Freescale rpmsg i2s interface.

The rpmsg i2s is based on RPMSG that used communicating with M4 core,
which provides a synchronous audio interface that supports fullduplex
serial interfaces with frame synchronization such as I2S.

Required properties:

  - compatible		: Compatible list, contains "fsl,imx7ulp-rpmsg-i2s".

  - fsl,audioindex      : This is an index indicating the audio device index in
			  the M4 side.

Example:
rpmsg_i2s: rpmsg-i2s {
	compatible = "fsl,imx7ulp-rpmsg-i2s";
	/* the audio device index in m4 domain */
	fsl,audioindex = <0> ;
	status = "okay";
};