dt-bindings: add document of Rockchip power domains

* Rockchip Power Domains

Rockchip processors include support for multiple power domains which can be

powered up/down by software based on different application scenes to save power.

Required properties for power domain controller:

- compatible: Should be one of the following.

	"rockchip,rk3288-power-controller" - for RK3288 SoCs.

- #power-domain-cells: Number of cells in a power-domain specifier.

	Should be 1 for multiple PM domains.

- #address-cells: Should be 1.

- #size-cells: Should be 0.

Required properties for power domain sub nodes:

- reg: index of the power domain, should use macros in:

	"include/dt-bindings/power-domain/rk3288.h" - for RK3288 type power domain.

- clocks (optional): phandles to clocks which need to be enabled while power domain

	switches state.

Example:

	power: power-controller {

		compatible = "rockchip,rk3288-power-controller";

		#power-domain-cells = <1>;

		#address-cells = <1>;

		#size-cells = <0>;

		pd_gpu {

			reg = <RK3288_PD_GPU>;

			clocks = <&cru ACLK_GPU>;

		};

	};

Node of a device using power domains must have a power-domains property,

containing a phandle to the power device node and an index specifying which

power domain to use.

The index should use macros in:

	"include/dt-bindings/power-domain/rk3288.h" - for rk3288 type power domain.

Example of the node using power domain:

	node {

		/* ... */

		power-domains = <&power RK3288_PD_GPU>;

		/* ... */

	};