iommu/arm-smmu: use outer cacheability for coherent table walks

Currently we're setting page table cacheability to "inner shareable"
when coherent table walking is enabled, but it should actually be "outer
shareable".  Fix this.

Change-Id: I7939bb75da8327f01a2fcd941a0d8358de6f0bc5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>