dt-bindings: pwm: stm32-lp: Document pin control sleep state

			bindings defined in pwm.txt.

Optional properties:

- pinctrl-names: 	Set to "default".

- pinctrl-0: 		Phandle pointing to pin configuration node for PWM.

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

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

- pinctrl-n: 		Phandle(s) pointing to pin configuration node for PWM,

			respectively for "default" and "sleep" states.

Example:

	timer@40002400 {

		pwm {

			compatible = "st,stm32-pwm-lp";

			#pwm-cells = <3>;

			pinctrl-names = "default";

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

			pinctrl-0 = <&lppwm1_pins>;

			pinctrl-1 = <&lppwm1_sleep_pins>;

		};

	};