ARM: dts: stm32: add PWM and triggers on stm32mp157c-ev1 board

stm32mp157c evaluation board has TIM2_CH4, TIM8_CH4 and TIM12_CH1
available on GPIO expansion connector.
Add PWM and associated triggers (for ADC/DAC) on these timers.
Keep them disabled so these pins can be used as GPIOs by default.

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