QTI Snapdragon Display Engine (SDE) shared plane

This driver allows to create multiple virtual planes from the same
physical plane. Only one plane can be activated at a time.

Required properties:
- compatible:		"qcom,sde-shared-plane"
- qcom,add-planes:	Shared planes to add to the DRM system. Each child
			represents a new plane, which has the properties of:
	qcom,plane-parent: required string, name of parent plane
			where this plane is created from.
	qcom,plane-name: required string, name of the new plane.
	qcom,plane-virt-name: optional string, name of the new multi-rect plane.
	qcom,plane-init-handoff: optional boolean, if set, the plane is inited
			to handoff state at system bootup.
	qcom,plane-init-active: optional boolean, if set, the plane is inited to
			active state at system bootup.
	qcom,plane-detach-handoff: optional boolean, if set, the plane is set to
			handoff state whenever it is detached.


Example:

/ {
	...

	qcom,sde-shared-plane {
		compatible = "qcom,sde-shared-plane";
		qcom,add-planes {
			plane@0 {
				qcom,plane-name = "plane-0-splash";
				qcom,plane-parent = "plane-0";
			};
			plane@1 {
				qcom,plane-name = "plane-1-splash";
				qcom,plane-parent = "plane-1";
			};
		};
	};
};