ARM: dts: stm32: add support for PWM on stm32mp157a-dk1

Add PWM support on stm32mp157a-dk1 board. There are several timers channels
made available on GPIO expansion and arduino connectors:
- Add PWM and trigger support (these timers can also be used as trigger
  for ADC).
It's easier then to configure them all. But keep them disabled by default,
so the pins are kept in their initial state to lower power consumption.
This way they can also be used as GPIO.

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