dt-bindings: add bindings for Rockchip grf

Add devicetree bindings for Rockchip grf which found on
Rockchip SoCs.
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

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

+* Rockchip General Register Files (GRF)
+
+The general register file will be used to do static set by software, which
+is composed of many registers for system control.
+
+From RK3368 SoCs, the GRF is divided into two sections,
+- GRF, used for general non-secure system,
+- PMUGRF, used for always on system
+
+Required Properties:
+
+- compatible: GRF should be one of the followings
+   - "rockchip,rk3066-grf", "syscon": for rk3066
+   - "rockchip,rk3188-grf", "syscon": for rk3188
+   - "rockchip,rk3228-grf", "syscon": for rk3228
+   - "rockchip,rk3288-grf", "syscon": for rk3288
+   - "rockchip,rk3368-grf", "syscon": for rk3368
+   - "rockchip,rk3399-grf", "syscon": for rk3399
+- compatible: PMUGRF should be one of the followings
+   - "rockchip,rk3368-pmugrf", "syscon": for rk3368
+   - "rockchip,rk3399-pmugrf", "syscon": for rk3399
+- reg: physical base address of the controller and length of memory mapped
+  region.
+
+Example: GRF and PMUGRF of RK3399 SoCs
+
+	pmugrf: syscon@ff320000 {
+		compatible = "rockchip,rk3399-pmugrf", "syscon";
+		reg = <0x0 0xff320000 0x0 0x1000>;
+	};
+
+	grf: syscon@ff770000 {
+		compatible = "rockchip,rk3399-grf", "syscon";
+		reg = <0x0 0xff770000 0x0 0x10000>;
+	};