DT: hwspinlock: add the CSR atlas7 hwspinlock bindings document

The Hardware Spinlock device on atlas7 provides hardware assistance
for synchronization between the multiple processors in the system
(dual Cortex-A7, CAN bus Cortex-M3 and audio DSP).
This patch adds the DT bindings information for this hwspinlock
module.

Reviewed-by: Suman Anna <[e-mail]>
Reviewed-by: Bjorn Andersson <[e-mail]>
Signed-off-by: Wei Chen <[e-mail]>
Signed-off-by: Barry Song <[e-mail]>
Signed-off-by: Ohad Ben-Cohen <[e-mail]>

Documentation/devicetree/bindings/hwlock/sirf,hwspinlock.txt

···
+SIRF Hardware spinlock device Binding
+-----------------------------------------------
+
+Required properties :
+- compatible : shall contain only one of the following:
+	"sirf,hwspinlock"
+
+- reg : the register address of hwspinlock
+
+- #hwlock-cells : hwlock users only use the hwlock id to represent a specific
+	hwlock, so the number of cells should be <1> here.
+
+Please look at the generic hwlock binding for usage information for consumers,
+"Documentation/devicetree/bindings/hwlock/hwlock.txt"
+
+Example of hwlock provider:
+	hwlock {
+		compatible = "sirf,hwspinlock";
+		reg = <0x13240000 0x00010000>;
+		#hwlock-cells = <1>;
+	};
+
+Example of hwlock users:
+	node {
+		...
+		hwlocks = <&hwlock 2>;
+		...
+	};