Freescale Synchronous Serial Interface

The SSI is a serial device that communicates with audio codecs.  It can
be programmed in AC97, I2S, left-justified, or right-justified modes.

Required properties:
- compatible:       Compatible list, contains "fsl,ssi".
- cell-index:       The SSI, <0> = SSI1, <1> = SSI2, and so on.
- reg:              Offset and length of the register set for the device.
- interrupts:       <a b> where a is the interrupt number and b is a
                    field that represents an encoding of the sense and
                    level information for the interrupt.  This should be
                    encoded based on the information in section 2)
                    depending on the type of interrupt controller you
                    have.
- interrupt-parent: The phandle for the interrupt controller that
                    services interrupts for this device.
- fsl,mode:         The operating mode for the SSI interface.
                    "i2s-slave" - I2S mode, SSI is clock slave
                    "i2s-master" - I2S mode, SSI is clock master
                    "lj-slave" - left-justified mode, SSI is clock slave
                    "lj-master" - l.j. mode, SSI is clock master
                    "rj-slave" - right-justified mode, SSI is clock slave
                    "rj-master" - r.j., SSI is clock master
                    "ac97-slave" - AC97 mode, SSI is clock slave
                    "ac97-master" - AC97 mode, SSI is clock master
- fsl,playback-dma: Phandle to a node for the DMA channel to use for
                    playback of audio.  This is typically dictated by SOC
                    design.  See the notes below.
- fsl,capture-dma:  Phandle to a node for the DMA channel to use for
                    capture (recording) of audio.  This is typically dictated
                    by SOC design.  See the notes below.
- fsl,fifo-depth:   The number of elements in the transmit and receive FIFOs.
                    This number is the maximum allowed value for SFCSR[TFWM0].
- fsl,ssi-asynchronous:
                    If specified, the SSI is to be programmed in asynchronous
                    mode.  In this mode, pins SRCK, STCK, SRFS, and STFS must
                    all be connected to valid signals.  In synchronous mode,
                    SRCK and SRFS are ignored.  Asynchronous mode allows
                    playback and capture to use different sample sizes and
                    sample rates.  Some drivers may require that SRCK and STCK
                    be connected together, and SRFS and STFS be connected
                    together.  This would still allow different sample sizes,
                    but not different sample rates.

Optional properties:
- codec-handle:     Phandle to a 'codec' node that defines an audio
                    codec connected to this SSI.  This node is typically
                    a child of an I2C or other control node.

Child 'codec' node required properties:
- compatible:       Compatible list, contains the name of the codec

Child 'codec' node optional properties:
- clock-frequency:  The frequency of the input clock, which typically comes
                    from an on-board dedicated oscillator.

Notes on fsl,playback-dma and fsl,capture-dma:

On SOCs that have an SSI, specific DMA channels are hard-wired for playback
and capture.  On the MPC8610, for example, SSI1 must use DMA channel 0 for
playback and DMA channel 1 for capture.  SSI2 must use DMA channel 2 for
playback and DMA channel 3 for capture.  The developer can choose which
DMA controller to use, but the channels themselves are hard-wired.  The
purpose of these two properties is to represent this hardware design.

The device tree nodes for the DMA channels that are referenced by
"fsl,playback-dma" and "fsl,capture-dma" must be marked as compatible with
"fsl,ssi-dma-channel".  The SOC-specific compatible string (e.g.
"fsl,mpc8610-dma-channel") can remain.  If these nodes are left as
"fsl,elo-dma-channel" or "fsl,eloplus-dma-channel", then the generic Elo DMA
drivers (fsldma) will attempt to use them, and it will conflict with the
sound drivers.