dt-bindings: pwm-stm32: Document pinctrl sleep state

Required parameters:

- compatible:		Must be "st,stm32-pwm".

- pinctrl-names: 	Set to "default".

- pinctrl-0: 		List of phandles pointing to pin configuration nodes for PWM module.

- pinctrl-names: 	Set to "default". An additional "sleep" state can be

			defined to set pins in sleep state when in low power.

- pinctrl-n: 		List of phandles pointing to pin configuration nodes for PWM module.

			For Pinctrl properties see ../pinctrl/pinctrl-bindings.txt

- #pwm-cells:		Should be set to 3. This PWM chip uses the default 3 cells

			bindings defined in pwm.txt.

			compatible = "st,stm32-pwm";

			#pwm-cells = <3>;

			pinctrl-0	= <&pwm1_pins>;

			pinctrl-names	= "default";

			pinctrl-1	= <&pwm1_sleep_pins>;

			pinctrl-names	= "default", "sleep";

			st,breakinput = <0 1 5>;

		};

	};