* SPEAr Shared IRQ layer (shirq)

SPEAr3xx architecture includes shared/multiplexed irqs for certain set
of devices. The multiplexor provides a single interrupt to parent
interrupt controller (VIC) on behalf of a group of devices.

There can be multiple groups available on SPEAr3xx variants but not
exceeding 4. The number of devices in a group can differ, further they
may share same set of status/mask registers spanning across different
bit masks. Also in some cases the group may not have enable or other
registers. This makes software little complex.

A single node in the device tree is used to describe the shared
interrupt multiplexor (one node for all groups). A group in the
interrupt controller shares config/control registers with other groups.
For example, a 32-bit interrupt enable/disable config register can
accommodate up to 4 interrupt groups.

Required properties:
  - compatible: should be, either of
     - "st,spear300-shirq"
     - "st,spear310-shirq"
     - "st,spear320-shirq"
  - interrupt-controller: Identifies the node as an interrupt controller.
  - #interrupt-cells: should be <1> which basically contains the offset
    (starting from 0) of interrupts for all the groups.
  - reg: Base address and size of shirq registers.
  - interrupts: The list of interrupts generated by the groups which are
    then connected to a parent interrupt controller. Each group is
    associated with one of the interrupts, hence number of interrupts (to
    parent) is equal to number of groups. The format of the interrupt
    specifier depends in the interrupt parent controller.

  Optional properties:
  - interrupt-parent: pHandle of the parent interrupt controller, if not
    inherited from the parent node.

Example:

The following is an example from the SPEAr320 SoC dtsi file.

shirq: interrupt-controller@0xb3000000 {
	compatible = "st,spear320-shirq";
	reg = <0xb3000000 0x1000>;
	interrupts = <28 29 30 1>;
	#interrupt-cells = <1>;
	interrupt-controller;
};