board: dhelectronics: stm32mp1: convert to livetree

Replace call to fdt_*() functions and access to gd->fdt_blob
with call to ofnode_*() functions to support a live tree.

Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>