dt-bindings: pwm-stm32-lp: Add #pwm-cells

STM32 Low-Power Timer supports generic 3 cells PWM to encode PWM number,
period and polarity.
Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

@@ -7,6 +7,8 @@ See ../mfd/stm32-lptimer.txt for details about the parent node.
 
 Required parameters:
 - compatible:		Must be "st,stm32-pwm-lp".
+- #pwm-cells:		Should be set to 3. This PWM chip uses the default 3 cells
+			bindings defined in pwm.txt.
 
 Optional properties:
 - pinctrl-names: 	Set to "default".
@@ -18,6 +20,7 @@ Example:
 		...
 		pwm {
 			compatible = "st,stm32-pwm-lp";
+			#pwm-cells = <3>;
 			pinctrl-names = "default";
 			pinctrl-0 = <&lppwm1_pins>;
 		};