dt-bindings: Add bindings for rk3308 clock controller

Add devicetree bindings for Rockchip cru which found on
Rockchip SoCs.
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/clock/rockchip,rk3308-cru.txt

+* Rockchip RK3308 Clock and Reset Unit
+
+The RK3308 clock controller generates and supplies clock to various
+controllers within the SoC and also implements a reset controller for SoC
+peripherals.
+
+Required Properties:
+
+- compatible: CRU should be "rockchip,rk3308-cru"
+- reg: physical base address of the controller and length of memory mapped
+  region.
+- #clock-cells: should be 1.
+- #reset-cells: should be 1.
+
+Optional Properties:
+
+- rockchip,grf: phandle to the syscon managing the "general register files"
+  If missing, pll rates are not changeable, due to the missing pll lock status.
+
+Each clock is assigned an identifier and client nodes can use this identifier
+to specify the clock which they consume. All available clocks are defined as
+preprocessor macros in the dt-bindings/clock/rk3308-cru.h headers and can be
+used in device tree sources. Similar macros exist for the reset sources in
+these files.
+
+External clocks:
+
+There are several clocks that are generated outside the SoC. It is expected
+that they are defined using standard clock bindings with following
+clock-output-names:
+ - "xin24m" - crystal input - required,
+ - "xin32k" - rtc clock - optional,
+ - "mclk_i2s0_8ch_in", "mclk_i2s1_8ch_in", "mclk_i2s2_8ch_in",
+   "mclk_i2s3_8ch_in", "mclk_i2s0_2ch_in",
+   "mclk_i2s1_2ch_in" - external I2S or SPDIF clock - optional,
+ - "mac_clkin" - external MAC clock - optional
+
+Example: Clock controller node:
+
+	cru: clock-controller@ff500000 {
+		compatible = "rockchip,rk3308-cru";
+		reg = <0x0 0xff500000 0x0 0x1000>;
+		rockchip,grf = <&grf>;
+		#clock-cells = <1>;
+		#reset-cells = <1>;
+	};
+
+Example: UART controller node that consumes the clock generated by the clock
+  controller:
+
+	uart0: serial@ff0a0000 {
+		compatible = "rockchip,rk3308-uart", "snps,dw-apb-uart";
+		reg = <0x0 0xff0a0000 0x0 0x100>;
+		interrupts = <GIC_SPI 18 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&cru SCLK_UART0>, <&cru PCLK_UART0>;
+		clock-names = "baudclk", "apb_pclk";
+		reg-shift = <2>;
+		reg-io-width = <4>;
+		status = "disabled";
+	};