arm: dma-mapping: add iova size alignment check

On 32bit systems, bitmap storing the iova information is stored in per
page and each page represents the 128MB of IOVA. So, for the
alloc_iova() to return the requested range, iova size should be aligned
to 128MB.

Change-Id: I66eb15cdbb3d6730cc4d84cad9f0c652f91278d6
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>