Subsystem Notification Virtual Driver

The guest VM uses this driver to communicate
subsystem state notifications to a backend driver
via the virtual device's registers.

[Root level node]
Required Properties:
-compatible : Should be "qcom,subsys-notif-virt"
-reg : The start and size of the virtual device's
	register set.
-reg-names :  Should be "vdev_base" for virtual device's
	base address.

[Child nodes]
-subsys-names : The name of the subsystem that the
	driver is registering to notifications for.
-offset : The offset from the virtual device's register
	base where the subsystem state will be written.
-type : The type of the subsystem.
	"virtual" - When the subsystem is loaded by the host VM
	"native" - When the subsystem is loaded by the guest VM

Required Property for "virtual" subsystem types:
-interrupts : Tuple defining the interrupt which the driver must
		register for to receive subsystem state notifications
		from the backend.
-interrupt-names: Must be "state-irq"

Example:

	subsys_notif_virt: qcom,subsys_notif_virt@2D000000 {
		compatible = "qcom,subsys-notif-virt";
		reg = <0x2D000000 0x400>;
		reg-names = "vdev_base";
		adsp {
			subsys-name = "adsp";
			interrupts = <0 43 0>;
			interrupt-names = "state-irq";
			type = "virtual";
			offset = <0>;
		};
		mpss {
			subsys-name = "modem";
			type = "native";
			offset = <256>;
		};
	};