spi: Add DT bindings documentation for Netlogic XLP SPI controller

Add DT bindings documentation for SPI controller driver used by
Netlogic XLP MIPS64 SoCs.
Signed-off-by: NKamlakant Patel <kamlakant.patel@broadcom.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

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

+SPI Master controller for Netlogic XLP MIPS64 SOCs
+==================================================
+
+Currently this SPI controller driver is supported for the following
+Netlogic XLP SoCs:
+	XLP832, XLP316, XLP208, XLP980, XLP532
+
+Required properties:
+- compatible		: Should be "netlogic,xlp832-spi".
+- #address-cells	: Number of cells required to define a chip select address
+			  on the SPI bus.
+- #size-cells		: Should be zero.
+- reg			: Should contain register location and length.
+- clocks		: Phandle of the spi clock
+- interrupts		: Interrupt number used by this controller.
+- interrupt-parent	: Phandle of the parent interrupt controller.
+
+SPI slave nodes must be children of the SPI master node and can contain
+properties described in Documentation/devicetree/bindings/spi/spi-bus.txt.
+
+Example:
+
+	spi: xlp_spi@3a100 {
+		compatible = "netlogic,xlp832-spi";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		reg = <0 0x3a100 0x100>;
+		clocks = <&spi_clk>;
+		interrupts = <34>;
+		interrupt-parent = <&pic>;
+
+		spi_nor@1 {
+			compatible = "spansion,s25sl12801";
+			#address-cells = <1>;
+			#size-cells = <1>;
+			reg = <1>;	/* Chip Select */
+			spi-max-frequency = <40000000>;
+		};
+};