Documentation: sound: Add documentation for new soundwire driver

Qualcomm Technologies, Inc. SoundWire Master controller

* swr_master

Required properties:

- compatible : should be "qcom,swr-mstr"

- #address-cells: must be 2

- #size-cells: must be 0

- qcom,swr-num-ports: number of ports in the master.

- qcom,swr-port-mapping: contains codec port_type and

	corresponding ch-mask entries for possible port types

	of the master port.

* wsa881x

Required properties:

- compatible : should be "qcom,wsa881x"

- reg : Unique device ID(48 bits) of Soundwire slave device node.

	In the below example, wsa881x is soundwire slave device for

	which the swr-devid is <0x0 0x032000> where 0x03 represents

	device Unique_ID, 0x20 represents Part_Id1 and 0x00

	represents part_Id2.

 Example:

swr0: swr_master {

		compatible = "qcom,swr-mstr";

		#address-cells = <2>;

		#size-cells = <0>;

		qcom,swr-num-ports = <8>;

		qcom,swr-port-mapping = <1 SPKR_L 0x1>,

		<2 SPKR_L_COMP 0xF>, <3 SPKR_L_BOOST 0x3>,

		<4 SPKR_R 0x1>, <5 SPKR_R_COMP 0xF>,

		<6 SPKR_R_BOOST 0x3>, <7 SPKR_L_VI 0x3>,

		<8 SPKR_R_VI 0x3>;

		wsa881x@32000 {

			compatible = "qcom,wsa881x";

			reg = <0x00 0x032000>;

		};

		wsa881x@42000 {

			compatible = "qcom,wsa881x";

			reg = <0x00 0x042000>;

		};

	};