ARM: dts: stm32: add stpmic1 support on stm32mp157a dk1 board

This patch adds stpmic1 support on stm32mp157a dk1 board.
The STPMIC1 is a PMIC from STMicroelectronics. The STPMIC1 integrates 10
regulators, 3 power switches, a watchdog and an input for a power on key.
The DMAs are disabled because the PMIC generates a very few traffic and
DMA channels may lack for other usage.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>