dt-bindings: Document the STM32 timer bindings

. STMicroelectronics STM32 timer

The STM32 MCUs family has several general-purpose 16 and 32 bits timers.

Required properties:

- compatible : Should be "st,stm32-timer"

- reg : Address and length of the register set

- clocks : Reference on the timer input clock

- interrupts : Reference to the timer interrupt

Optional properties:

- resets: Reference to a reset controller asserting the timer

Example:

timer5: timer@40000c00 {

	compatible = "st,stm32-timer";

	reg = <0x40000c00 0x400>;

	interrupts = <50>;

	resets = <&rrc 259>;

	clocks = <&clk_pmtr1>;

};