iommu/io-pgtable-arm: Use outer shareable for all configurations

Change over to using outer shareable for both coherent and
non-coherent page tables and for both coherent and non-coherent
data buffers.
This is done to be more in line with the ARM spec.

Change-Id: Icebf88641a5ebb82bb4b7577d1ab402580b1460c
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>