Documentation: dt: add the omap hwspinlock bindings document

HwSpinlock IP is present only on OMAP4 and other newer SoCs,
which are all device-tree boot only. This patch adds the
DT bindings information for OMAP hwspinlock module.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>

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

+OMAP4+ HwSpinlock Driver
+========================
+
+Required properties:
+- compatible:		Should be "ti,omap4-hwspinlock" for
+			    OMAP44xx, OMAP54xx, AM33xx, AM43xx, DRA7xx SoCs
+- reg:			Contains the hwspinlock module register address space
+			(base address and length)
+- ti,hwmods:		Name of the hwmod associated with the hwspinlock device
+- #hwlock-cells:	Should be 1. The OMAP hwspinlock users will use a
+			0-indexed relative hwlock number as the argument
+			specifier value for requesting a specific hwspinlock
+			within a hwspinlock bank.
+
+Please look at the generic hwlock binding for usage information for consumers,
+"Documentation/devicetree/bindings/hwlock/hwlock.txt"
+
+Example:
+
+/* OMAP4 */
+hwspinlock: spinlock@4a0f6000 {
+	compatible = "ti,omap4-hwspinlock";
+	reg = <0x4a0f6000 0x1000>;
+	ti,hwmods = "spinlock";
+	#hwlock-cells = <1>;
+};