dt-bindings: modify document of Rockchip power domains

Required properties for power domain controller:

- compatible: Should be one of the following.

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

	"rockchip,rk3368-power-controller" - for RK3368 SoCs.

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

	Should be 1 for multiple PM domains.

- #address-cells: Should be 1.

Required properties for power domain sub nodes:

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

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

	"include/dt-bindings/power/rk3368-power.h" - for RK3368 type power domain.

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

	switches state.

		};

	};

	 power: power-controller {

                compatible = "rockchip,rk3368-power-controller";

                #power-domain-cells = <1>;

                #address-cells = <1>;

                #size-cells = <0>;

                pd_gpu_1 {

                        reg = <RK3368_PD_GPU_1>;

                        clocks = <&cru ACLK_GPU_CFG>;

                };

        };

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/rk3288-power.h" - for rk3288 type power domain.

	"include/dt-bindings/power/rk3368-power.h" - for rk3368 type power domain.

Example of the node using power domain:

		power-domains = <&power RK3288_PD_GPU>;

		/* ... */

	};

	node {

                /* ... */

                power-domains = <&power RK3368_PD_GPU_1>;

                /* ... */

        };