board: stm32mp1: convert to livetree

Replace gd->fdt_blob access with fdt_getprop() function to the
function ofnode_get_property() to support a live tree.

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