ARM: dts: stm32: add PWM pins used on stm32mp157c-ev1 board

stm32mp157c evaluation board has following PWM pins available on GPIO
expansion connector:
- TIM2_CH4 (PA3)
- TIM8_CH4 (PI2)
- TIM12_CH1 (PH6)

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