board: st: add generic board for STM32MP1 family

Add first support for STM32MP157C-ED1 board with "Basic" boot chain
1/ Boot Rom: load SPL with STM32 image header in SYSRAM
2/ SPL: power up and initialize the DDR and load U-Boot image
        from SDCARD in DDR
3/ U-Boot: search and load extlinux.conf in SDCARD
           (DISTRO activated)

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