iommu: Introduce DOMAIN_ATTR_ATOMIC

Clock and regulator operations may not be atomic; however iommu operations
are not required to follow this rule. This can be accomplished simply by
leaving clocks and regulators on when a domain is attached, but at a
power cost. Therefore, add a flag to indicate that a domain may be used
in atomic context to optimize domains which do not require this.

Change-Id: Ia6ca2690154441fcc4d1ca38bf288930f20f3dd1
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>