QVM Guest Shared Memory

guest_shm is a device that enables linux as a guest operating system
to allocate shared memory between virtual machines and send notifications
of updates to other virtual machines.

Required properties:
- compatible:		Must be "qvm,guest_shm".
- interrupt-parent:	Parent interrupt controller.
- interrupts:		Should contain QVM interrupt.
- reg:			Physical address of the guest factory and length.

Example:
	qvm,guest_shm {
		compatible = "qvm,guest_shm";
		interrupt-parent = <&gic>;
		interrupts = <6 4>;
		reg = <0x1c050000 0x1000>;
	};
