arm64/iommu: handle atomic pool addresses in ->get_sgtable and ->mmap

Atomic dma pool is remapped to vmalloc area by default. get_sgtable
and mmap on addresses from this pool returns an error now. Fix that.

Change-Id: I2a5f4f5447ffc6fa6ee3baee05a496110de40b3c
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>