dt-bindings: hwlock: Document STM32 hwspinlock bindings

STM32 Hardware Spinlock Device Binding

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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";

	};