WCD audio codec SPI driver support

* wcd_spi

The wcd_spi device can be added as child device node to the audio codec device
node if the audio codec has SPI slave hardware. The codec driver upon probe will
create spi_device and perform spi_add_device. Note that the SPI framework does
not parse this DT node and hence the DT properties defined by SPI framework
cannot be used in wcd_spi device node.

Required properties:

- compatible : "qcom,wcd-spi-v2"

- qcom,master-bus-num : Bus number of the SPI master controller driver. This
			will be used to get a reference to the SPI master.

- qcom,chip-select : Specifies the chip select number used for spi device
		     registration.

- qcom,max-frequency : Specifies the max frequency of the SPI interface.

- qcom,mem-base-addr : Defines the memory base address from the SPI
		       memory map. This will be used as an offset to read
		       and write memory.

Example:

tavil_codec {
	wcd_spi_0: wcd_spi {
		compatible = "qcom,wcd-spi-v2";
		qcom,master-bus-num = <10>;
		qcom,chip-select = <0>;
		qcom,max-frequency = <24000000>;
		qcom,mem-base-addr = <0x100000>;
	};
};
