configs: stm32mp15: Enable OF_BOARD flag

Since commit 98550343 ("fdt: Don't call board_fdt_blob_setup()
 without OF_BOARD") board_fdt_blob_setup() is no more called on
STM32MP platforms in trusted boot which hangs during boot process.

Enable OF_BOARD flag to fix this issue.

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