board: stm32mp1: use FDT address provided by TF-A at boot time

board: stm32mp1: use FDT address provided by TF-A at boot time

Save and use the FDT address provided by TF-A in r2 at boot time
(it is NT_FW_CONFIG = Non Trusted Firmware configuration file)

Address is saved in save_boot_params(), called by start.S
and the used DTB is gd->fdt_blob = board_fdt_blob_setup().

If dtb is not provided or invalid, U-Boot use as fallback
the builtin DTB.

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