dt-bindings: hwlock: Document STM32 hwspinlock bindings

Add bindings for STM32 hardware spinlock device
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/devicetree/bindings/hwlock/st,stm32-hwspinlock.txt

+STM32 Hardware Spinlock Device Binding
+-------------------------------------
+
+Required properties :
+- compatible : should be "st,stm32-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.
+- clock-names : Must contain "hsem".
+- 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@4c000000 {
+		compatible = "st,stm32-hwspinlock";
+		#hwlock-cells = <1>;
+		reg = <0x4c000000 0x400>;
+		clocks = <&rcc HSEM>;
+		clock-names = "hsem";
+	};