spi: xilinx: Add devicetree binding for spi-xilinx

Xilinx SPI controller Device Tree Bindings

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Required properties:

- compatible		: Should be "xlnx,xps-spi-2.00.a" or "xlnx,xps-spi-2.00.b"

- reg			: Physical base address and size of SPI registers map.

- interrupts		: Property with a value describing the interrupt

			  number.

- interrupt-parent	: Must be core interrupt controller

Optional properties:

- xlnx,num-ss-bits	: Number of chip selects used.

Example:

	axi_quad_spi@41e00000 {

			compatible = "xlnx,xps-spi-2.00.a";

			interrupt-parent = <&intc>;

			interrupts = <0 31 1>;

			reg = <0x41e00000 0x10000>;

			xlnx,num-ss-bits = <0x1>;

	};