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

Add documentation for pin control sleep state on STM32 LPTimer PWM.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-stm32-lp.txt

@@ -11,8 +11,10 @@ Required parameters:
 			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 {
@@ -21,7 +23,8 @@ Example:
 		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>;
 		};
 	};