iommu/arm-smmu: Add support for map/unmap to be atomic

Certain clients need map and unmap operations to be in the atomic
context.  This currently doesn't work since we need to prepare clocks
and enable regulators (for tlb invalidation), and take a mutex (to
protect the smmu_domain->smmu instance), which are all sleeping
operations.

Fix this by introducing a domain attribute to indicate that map and
unmap should be atomic.  When this domain attribute is set we assume:

  (1) clocks are already prepared
  (2) regulators are already on
  (3) the domain is attached to an SMMU

(1) and (2) allow us to skip preparing clocks and enabling regulators
during map and unmap and are taken care of internally in the
driver.  (3) is a contract with clients that allows us to skip taking
the mutex that protects smmu_domain->smmu since we assume that the SMMU
instance is valid and will not be going away.  If clients break this
contract then there are race conditions all over the place that will
eventually result in a NULL dereference (specifically, of
smmu_domain->smmu).

Change-Id: I3e21df02f7b0cd116fb558715eea16f2119f1d03
Signed-off-by: Neeti Desai <neetid@codeaurora.org>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>