dt-bindings: i2c: Add Spreadtrum I2C controller documentation

I2C for Spreadtrum platforms

Required properties:

- compatible: Should be "sprd,sc9860-i2c".

- reg: Specify the physical base address of the controller and length

  of memory mapped region.

- interrupts: Should contain I2C interrupt.

- clock-names: Should contain following entries:

  "i2c" for I2C clock,

  "source" for I2C source (parent) clock,

  "enable" for I2C module enable clock.

- clocks: Should contain a clock specifier for each entry in clock-names.

- clock-frequency: Constains desired I2C bus clock frequency in Hz.

- #address-cells: Should be 1 to describe address cells for I2C device address.

- #size-cells: Should be 0 means no size cell for I2C device address.

Optional properties:

- Child nodes conforming to I2C bus binding

Examples:

i2c0: i2c@70500000 {

	compatible = "sprd,sc9860-i2c";

	reg = <0 0x70500000 0 0x1000>;

	interrupts = <GIC_SPI 11 IRQ_TYPE_LEVEL_HIGH>;

	clock-names = "i2c", "source", "enable";

	clocks = <&clk_i2c3>, <&ext_26m>, <&clk_ap_apb_gates 11>;

	clock-frequency = <400000>;

	#address-cells = <1>;

	#size-cells = <0>;

};