The bounds check in ARM_CASSERT_MMAP does not take into account the
array sentinel in plat_arm_mmap. This commit fixes this, and adds an
additional check to ensure the number of entries in the array is
within the bounds of PLAT_ARM_MMAP_ENTRIES.

Change-Id: Ie6df10c0aa0890d62826bc3224ad7b3e36fd53e2
Signed-off-by: Chris Kay <chris.kay@arm.com>