arm: keep address range pmd aligned while remap

During early init, all dma areas are remapped to PAGE_SIZE
granularity. Since full pmd regions are cleared to be
remapped into PAGE_SIZE, ensure that address range is pmd
size aligned while not crossing memory boundaries.

This would ensure that even if address region is not pmd
aligned, its mapping would not be cleared but factored in to
PAGE_SIZE regions.

Change-Id: Iad4ad7fd6169cdc693d532821aba453465addb7c
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>