stm32mp1: add eMMC support for ED1

Add command GPT support
Add EMMC boot support
Add the 2 other SDMMC instances for ED1:
- SDMMC2 = mmc 1, eMMC on the ED1 board
- SDMMC3 = extension connector, deactivated by default

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