feat(zynqmp): add hooks for mmap and early setup

Add early setup hooks (via custom_early_setup()) and provide a way
to cover custom memory mapping which includes extending memory map
via custom_mmap_add().

This likely also require to align MAX_XLAT_TABLE, MAX_XLAT_TABLES
macros. It can be done for example by defining these macros in
custom_pkg.mk
MAX_MMAP_REGIONS	:= XY
$(eval $(call add_define,MAX_MMAP_REGIONS))
MAX_XLAT_TABLES		:= XZ
$(eval $(call add_define,MAX_XLAT_TABLES))

custom_early_setup() can be used for early low level operations
related to setting up the system to correct state.

Signed-off-by: Amit Nagal <amit.nagal@amd.com>
Change-Id: I61df6f9ba5af0bc97c430974fb10a2edde44f23d