Device Tree bindings for Xilinx Zynq MPSoC PM domains

The binding for zynqmp-genpd follow the common generic PM domain binding[1].

[1] Documentation/devicetree/bindings/power/power_domain.txt

== Zynq MPSoC Generic PM Domain Node ==

Required properties:
 - compatible: Must be: "xlnx,zynqmp-genpd"

This node contains a number of subnodes, each representing a single PM domain
that PM domain consumer devices reference.

== PM Domain Nodes ==

Required properties:
 - #power-domain-cells: Number of cells in a PM domain specifier. Must be 0.
 - pd-id: List of domain identifiers of as defined by platform firmware. These
	  identifiers are passed to the PM firmware.

Example:
	zynqmp-genpd {
		compatible = "xlnx,zynqmp-genpd";

		pd_usb0: pd-usb0 {
			pd-id = <22>;
			#power-domain-cells = <0>;
		};

		pd_sata: pd-sata {
			pd-id = <25>;
			#power-domain-cells = <0>;
		};

		pd_gpu: pd-gpu {
			pd-id = <58 20 21>;
			#power-domain-cells = <0x0>;
		};
	};

	sata0: ahci@SATA_AHCI_HBA {
		...
		power-domains = <&pd_sata>;
		...
	};
