Having these definitions in board_arm_def.h forces to
all the arm platforms to use the same definition for
PLAT_ARM_MEM_PROT_ADDR.

This macro also enables the mem-protect mechanism,
which means that all the arm platform has enabled
mem-protect and they cannot get rid of it.

Change-Id: Id29d2c51cbe6edc15f010a9f5a20c42266c48a08
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>