iommu: Move away from gen_pool_alloc_aligned

gen_pool_alloc_aligned is a non standard API which needs
to be deprecated. gen_pool_alloc does not support aligned
allocations however. Move away from gen_pool_alloc_aligned
to just using a simple bitmap instead.

Change-Id: I4081ec12e0dc85f6911f2f95b7a4257e0d0debd5
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>