configs: stm32mp1: activate OF_LIVE for DT live support

Activate the live DT to reduce the DT parsing time.

For example the boot time is reduced by 200ms on STM32MP157C-EV1 board
for stm32mp15_basic_defconfig (boot with SPL) or
stm32mp15_trusted_defconfig (boot with TF-A).

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