spi: Add DT bindings documentation for Netlogic XLP SPI controller

SPI Master controller for Netlogic XLP MIPS64 SOCs

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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>;

		};

};