DT: hwspinlock: Add binding documentation for Spreadtrum hwspinlock

This patch adds the binding documentation for Spreadtrum hardware spinlock
device.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

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

+SPRD Hardware Spinlock Device Binding
+-------------------------------------
+
+Required properties :
+- compatible : should be "sprd,hwspinlock-r3p0".
+- 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.
+- clock-names : Must contain "enable".
+- clocks : Must contain a phandle entry for the clock in clock-names, see the
+	common clock bindings.
+
+Please look at the generic hwlock binding for usage information for consumers,
+"Documentation/devicetree/bindings/hwlock/hwlock.txt"
+
+Example of hwlock provider:
+	hwspinlock@40500000 {
+		compatible  = "sprd,hwspinlock-r3p0";
+		reg = <0 0x40500000 0 0x1000>;
+		#hwlock-cells = <1>;
+		clock-names = "enable";
+		clocks = <&clk_aon_apb_gates0 22>;
+	};