iommu: introduce DOMAIN_ATTR_CONTEXT_BANK

After attaching a domain, this attribute may be
queried to determine which hardware context bank
was assigned.

Change-Id: I31e674672041103007fcaff3f83a0cc2c33a4a6d
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>