iommu: msm: Cache whether the soc supports v0 IOMMUs

Currently, each call to determine if the SOC supports the v0
IOMMU makes at least one call to of_find_compatible_node. This
function is not excessively slow usually but
msm_soc_version_supports_iommu_v0 is called on the fast path which
introduces latency. Since v0 support is not something that can be
changed at runtime, cache the status of v0 support in a static variable
to avoid calling the devicetree functions frequently.

Change-Id: I835774a2cecb074f2bf25b1d3acb6fb87a5c0417
CRs-Fixed: 507473
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>