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.

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>