dt-bindings: Document the STM32 timer bindings

This adds documentation of device tree bindings for the
STM32 timer.

Tested-by: Chanwoo Choi <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Maxime Coquelin <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>

Documentation/devicetree/bindings/timer/st,stm32-timer.txt

···
+. 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>;
+};