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

Fixes: 61fc211c ("ARM: dts: stm32: add timers support to stm32mp157c")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>