ARM: 8740/1: NOMMU: Make sure we do not hold stale data in mem[] array

adjust_lowmem_bounds() called twice which can lead to stalled data
(i.e. subreg) value in mem[] array after the first call.

Zero out mem[] array before we allocate MPU regions for memory.

Fixes: 5c9d9a1b ("ARM: 8712/1: NOMMU: Use more MPU regions to cover memory")
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>