STM32 Timers support generic 3 cells PWM to encode PWM number, period
and polarity.

Fixes: 7edf7369 ("pwm: Add driver for STM32 plaftorm")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>