iommu: arm-smmu: Clarify DOMAIN_ATTR_FAST dependency

DOMAINA_ATTR_FAST depends on DOMAIN_ATTR_ATOMIC since it does not make
an explicit call to enable iommu clocks and power.

Move this dependency to the iommu_domain_set_attr() callback to prepare
for future changes.

Change-Id: Ia506d458b5b92af26782932d7c019045c1fc5177
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>