arm: dma-mapping: map_page map to nearest page

Since the page offset can be greater than the size of a page fix
arm_coherent_iommu_map_page so that it maps to nearest page
boundary.

This both prevents unnecessarily mapping memory we don't need to
map and fixes a bug where the unmap wasn't unmapping this extra
memory.

Change-Id: Iaa69aff7505ee75d1f2e69bb0cda814bc6211bd3
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>