DT: i2c: Add binding document for IMG I2C SCB

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>;

};