dt-bindings: add document of Rockchip power domains

This add the necessary binding documentation for the power domains
found on Rockchip SoCs.
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/soc/rockchip/power_domain.txt

+* 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>;
+		/* ... */
+	};