arm: ensure enough memory is available for movable zone

If CONFIG_HIGHMEM is enabled, the memory from the movable
zone will come from highmem as the zone of highest
address present is converted to ZONE_MOVABLE, and only
one zone may be used for ZONE_MOVABLE.

Ensure that ZONE_HIGHMEM is large enough for the
movable zone's required size to be created from
it if CONFIG_FIX_MOVABLE_ZONE is enabled as well.

There needs to be a small ZONE_HIGHMEM even after
memory hotremove (due to a generic 3.0 bug), so
extra is reserved for now.

Change-Id: I285c6ad24a6f2f18fc9e6510d379b126f201b082
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>