ARM: dts: stm32: add pwm cells to stm32f429

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

Fixes: c0e14fc7 ("ARM: dts: stm32: add Timers driver for stm32f429
MCU")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>