DT: hwspinlock: Add binding documentation for Qualcomm hwmutex

Add binding documentation for the Qualcomm Hardware Mutex.
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Reviewed-by: Andy Gross <agross@codeaurora.org>
Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>

Documentation/devicetree/bindings/hwlock/qcom-hwspinlock.txt

+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>;
+	};