pwm: stm32: LPTimer: Use 3 cells ->of_xlate()

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>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>