plat/arm/sgi: update mmap and xlat count

A single chip platform requires five mmap entries and a corresponding
number of translation tables. For every additional chip in the system,
three additional mmap entries are required to map the shared SRAM and
the IO regions. A corresponding number of additional translation
tables are required as well.

Change-Id: I1332a1305f2af62181387cf36954f6fb0e6f11ed
Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>