DT: i2c: Add binding document for IMG I2C SCB

Introduce a devicetree binding for Imagination Technologies
I2C SCB controller.
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-img-scb.txt

+IMG Serial Control Bus (SCB) I2C Controller
+
+Required Properties:
+- compatible: "img,scb-i2c"
+- reg: Physical base address and length of controller registers
+- interrupts: Interrupt number used by the controller
+- clocks : Should contain a clock specifier for each entry in clock-names
+- clock-names : Should contain the following entries:
+                "scb", for the SCB core clock.
+                "sys", for the system clock.
+- clock-frequency: The I2C bus frequency in Hz
+- #address-cells: Should be <1>
+- #size-cells: Should be <0>
+
+Example:
+
+i2c@18100000 {
+	compatible = "img,scb-i2c";
+	reg = <0x18100000 0x200>;
+	interrupts = <GIC_SHARED 2 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&i2c0_clk>, <&system_clk>;
+	clock-names = "scb", "sys";
+	clock-frequency = <400000>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+};