board: stm32mp1: support boot from spi-nand

Manage BOOT_FLASH_SPINAND, with boot_device="spi-nand"
and treat this value in bootcmd_stm32mp.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
5 files changed