dt-bindings: pwm: Add STM32 LPTimer PWM binding

Add documentation for STMicroelectronics STM32 Low-Power Timer
PWM binding.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

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

+STMicroelectronics STM32 Low-Power Timer PWM
+
+STM32 Low-Power Timer provides single channel PWM.
+
+Must be a sub-node of an STM32 Low-Power Timer device tree node.
+See ../mfd/stm32-lptimer.txt for details about the parent node.
+
+Required parameters:
+- compatible:		Must be "st,stm32-pwm-lp".
+
+Optional properties:
+- pinctrl-names: 	Set to "default".
+- pinctrl-0: 		Phandle pointing to pin configuration node for PWM.
+
+Example:
+	timer@40002400 {
+		compatible = "st,stm32-lptimer";
+		...
+		pwm {
+			compatible = "st,stm32-pwm-lp";
+			pinctrl-names = "default";
+			pinctrl-0 = <&lppwm1_pins>;
+		};
+	};