dt-bindings: pwm: stm32: Add #pwm-cells

STM32 Timers support generic 3 cells PWM bindings to encode PWM number,
period and polarity as defined in pwm.txt.

Fixes: cd9a99c2

 ("dt-bindings: pwm: Add STM32 bindings")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>