iommu/arm-smmu: add DT option to skip certain initialization

On some targets, context banks can be dedicated for use by other
execution environments outside of Linux.  However, we currently assume
that we own all of the context banks during SMMU initialization in
arm_smmu_device_reset.  This can mess up the other execution
environments since we are trampling on their SCTLR, SMR, and S2CR.
Provide a DT option to skip this initialization altogether, since the
other execution environment should have already initialized the SMMU
anyways.

Change-Id: I0ed4cbbcdad596f9201f83cb7d0e28a289e18a6e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>