iommu/arm-smmu: don't touch the secure STLBIALL register

Currently we do a STLBIALL when we initialize the SMMU.  However, in
some configurations that register is not supposed to be touched and is
marked as "Secure only" in the spec.  Rip it out.

Change-Id: I3a6e3f171f8f9fd82257270e3a7202ecbc8bd6ad
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>