iommu/io-pgtable-arm: Generate a fault on TTBR1 usage

TTBR1 shouldn't currently be used at all.  Any such usage is the result
of a bug.  Configure TCR such that any usage of TTBR1 will generate a
fault, which will help catch such bugs earlier.

Change-Id: I74f2dc9580e5aed5d391debe23c7f5cc9fc1f672
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>