dts: add device tree for STM32MP157C-ED1 board

Add minimal devicetree for STM32MP157C-ED1 board,
with only the devices to allow boot from SDCARD:
- RCC for clock and reset
- UART4 for console
- I2C and PMIC
- DDR
- SDMMC0 for SDCard

Waiting Kernel upstream for alignment.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>