Rework page table setup for varying number of mem regions

Change arm_setup_page_tables() to take a variable number of memory
regions. Remove coherent memory region from BL1, BL2 and BL2U as
their coherent memory region doesn't contain anything and
therefore has a size of 0. Add check to ensure this
doesn't change without us knowing.

Change-Id: I790054e3b20b056dda1043a4a67bd7ac2d6a3bc0
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>