dt-bindings: Add documentation for mpm irqchip

QTI MPM interrupt controller

MPM (MSM sleep Power Manager) is QTI's platform parent

interrupt controller. It manages subsystem wakeups and

resources during sleep. This driver marks the wakeup

interrupts in APSS such that it monitors the interrupts

when the system is asleep, wakes up the APSS when one

of these interrupts occur and replays it to the subsystem

interrupt controller after it becomes operational.

Platform interrupt controller MPM is next in hierarchy,

followed by others.

This defines 2 interrupt controllers to monitor the

interrupts when system is asleep:

One to monitor the wakeup capable gic interrupts called

wakegic.

Properties:

- compatible:

	Usage: required

	Value type: <string>

	Definition: Should contain "qcom,mpm-gic" and the respective

	target compatible flag from below ones.

		"qcom,mpm-gic-holi"

- interrupts:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: should specify the IRQ used by remote processor to

	wakeup APSS.

- interrupt-parent:

	Usage: required

	Value type: <phandle>

	Definition: Specifies the interrupt parent necessary for

	hierarchical domain to operate.

- interrupt-controller:

	Usage: required

	Value type: <bool>

	Definition: Identifies the node as an interrupt controller.

- reg:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: Specifies the base physical address to trigger an

	interrupt into remote processor.

-reg-names:

	Usage: required

	Value type: <string>, <string>

	Definition: Specifies the address field names.

- qcom,num-mpm-irqs:

	Usage: optional

	Value type: <value>

	Defination: Specifies the number of interrupts supported.

Example:

wakegic: wake-gic@7781b8 {

	compatible = "qcom,mpm", "qcom,mpm-gic-holi";

	interrupts = <GIC_SPI 171 IRQ_TYPE_EDGE_RISING>;

	reg = <0x601d4 0x1000>,

	    <0xb011008 0x4>;  /* MSM_APCS_GCC_BASE 4K */

	reg-names = "vmpm", "ipc";

	interrupt-controller;

	interrupt-parent = <&intc>;

	#interrupt-cells = <3>;

};