iommu/io-pgtable-arm: Make cacheable mappings outer-shareable

Cacheable mappings should be inner- and outer-shareable, but
currently we're only making them inner-shareable.  Make them
outer-shareable as well.

Change-Id: I50e0ee3654d5d13dcbfdbb31d2df4d56b79a2b78
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>