dt-bindings: i2c: Add Spreadtrum I2C controller documentation

This patch adds the binding documentation for Spreadtrum I2C
controller device.
Signed-off-by: NBaolin Wang <baolin.wang@spreadtrum.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-sprd.txt

+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>;
+};
+