arm: mm: Account for lowmem in the static vm

Linux page directory size differs from the ARM section size used
for lowmem mappings (2MB vs. 1MB). The ioremap code takes this
into account when static mappings are set up and pads PMD sections
accordingly. Lowmem needs to be tracked in the static mappings as
well to account for the size difference as well. If the lowmem
is not tracked, we may attempt to use part of an address range that
should be reserved which can trigger BUGs() or unexpected faults.

Change-Id: I60b1ce50a7cafbd189066d7331ee8b4c4ce3027b
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>