iommu: Add DOMAIN_ATTR_FAST for requesting a fast domain

Some IOMMU drivers offer "fast" page table management routines for
special cases.  There is often a trade-off with memory, etc. with these
so make their usage explicit with a domain attribute.

CRs-Fixed: 997751
Change-Id: Ia9f8ad6d924b294b6758970da2e9767f183b5649
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>