arm64: Allow clients to use bitmap iova allocator

With the dma_map_ops defaulted to upstream ops, allow iommu
clients to revert back to previous functioning bitmap iova
allocator just in case, if the upstream dma ops starts to behave
improperly.

Change-Id: I21d1b4a882f0d6905b008e419b5d3a3e6446a4f2
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>