ARM: dts: stm32: Add LPtimer support to stm32mp157c

Add LPtimer definitions, depending on features they provide:
- lptimer1 & 2 can act as PWM, trigger and encoder/counter
- lptimer3 can act as PWM and trigger
- lptimer4 & 5 can act as PWM

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