DT: hwspinlock: Add binding documentation for Qualcomm hwmutex

Qualcomm Hardware Mutex Block:

The hardware block provides mutexes utilized between different processors on

the SoC as part of the communication protocol used by these processors.

- compatible:

	Usage: required

	Value type: <string>

	Definition: must be one of:

		    "qcom,sfpb-mutex",

		    "qcom,tcsr-mutex"

- syscon:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: one cell containing:

		    syscon phandle

		    offset of the hwmutex block within the syscon

		    stride of the hwmutex registers

- #hwlock-cells:

	Usage: required

	Value type: <u32>

	Definition: must be 1, the specified cell represent the lock id

		    (hwlock standard property, see hwlock.txt)

Example:

	tcsr_mutex_block: syscon@fd484000 {

		compatible = "syscon";

		reg = <0xfd484000 0x2000>;

	};

	hwlock@fd484000 {

		compatible = "qcom,tcsr-mutex";

		syscon = <&tcsr_mutex_block 0 0x80>;

		#hwlock-cells = <1>;

	};