Binding for Qualcomm Atheros AR7xxx/AR9xxx SPI controller

Required properties:
- compatible: has to be "qca,<soc-type>-spi", "qca,ar7100-spi" as fallback.
- reg: Base address and size of the controllers memory area
- clocks: phandle of the AHB clock.
- clock-names: has to be "ahb".
- #address-cells: <1>, as required by generic SPI binding.
- #size-cells: <0>, also as required by generic SPI binding.

Child nodes as per the generic SPI binding.

Example:

	spi@1f000000 {
		compatible = "qca,ar9132-spi", "qca,ar7100-spi";
		reg = <0x1f000000 0x10>;

		clocks = <&pll 2>;
		clock-names = "ahb";

		#address-cells = <1>;
		#size-cells = <0>;
		m25p80@0 {
			#address-cells = <1>;
			#size-cells = <1>;
			compatible = "n25q128a11";
			linux,modalias = "m25p80", "n25q128a11";
			reg = <0>;
			spi-max-frequency = <25000000>;
		};
		mt29f@1 {
			#address-cells = <1>;
			#size-cells = <1>;
			compatible = "spinand,mt29f";
			reg = <1>;
			spi-max-frequency = <50000000>;
			status = "okay";
		};
	};