iommu: Introduce DOMAIN_ATTR_SPLIT_TABLES

Introduce DOMAIN_ATTR_SPLIT_TABLES for IOMMU domains that will
use the upper portion of the IOVA space, in conjunction with the
lower portion of the IOVA space.

Change-Id: I9d786396f5306f920c1d023bba8c85dce041c83c
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>