feat(stm32mp1): configure the serial boot load address

feat(stm32mp1): configure the serial boot load address

For product with 128MB DDR size, the OP-TEE is located at the end
of the DDR and the FIP can't be loaded at the default location
because it overlap the OP-TEE final location. So the default value
for DWL_BUFFER_BASE is invalid.

To avoid this conflict the serial boot load address = DWL_BUFFER_BASE
can be modified with a configuration flags.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: Ie27b87c10c57fea5d4c6200ce4f624e775b9a080