dt-bindings: pwm: Add STM32 LPTimer PWM binding

Add documentation for STMicroelectronics STM32 Low-Power Timer
PWM binding.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>