iommu: arm-smmu: Make smr mask discovery work with static cb

SMR0 may be inuse when the arm smmu driver probes. Therefore attempt
to find another context bank to use instead of trampling on the
existing settings.

Change-Id: I264d987a9801019405b3a9cfb65d546bf89b011e
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>