spi: Add Spreadtrum SPI controller documentation

This patch adds the binding documentation for Spreadtrum SPI
controller device.
Signed-off-by: Lanqing Liu <lanqing.liu@spreadtrum.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

+Spreadtrum SPI Controller
+
+Required properties:
+- compatible: Should be "sprd,sc9860-spi".
+- reg: Offset and length of SPI controller register space.
+- interrupts: Should contain SPI interrupt.
+- clock-names: Should contain following entries:
+	"spi" for SPI clock,
+	"source" for SPI source (parent) clock,
+	"enable" for SPI module enable clock.
+- clocks: List of clock input name strings sorted in the same order
+	as the clock-names property.
+- #address-cells: The number of cells required to define a chip select
+	address on the SPI bus. Should be set to 1.
+- #size-cells: Should be set to 0.
+
+Example:
+spi0: spi@70a00000{
+	compatible = "sprd,sc9860-spi";
+	reg = <0 0x70a00000 0 0x1000>;
+	interrupts = <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>;
+	clock-names = "spi", "source","enable";
+	clocks = <&clk_spi0>, <&ext_26m>, <&clk_ap_apb_gates 5>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+};