iommu/arm-smmu: Remove DOMAIN_ATTR_DEBUG

DOMAIN_ATTR_DEBUG is queried by the qsmmuv500_iova_to_phys()
function, which will move outside of arm-smmu.c, requiring
arm_smmu_domain_get_attr() to be exposed, which doesn't make
sense for only one attribute.

Thus, remove this domain attribute and inform the SMMU driver
that it is operating on a debug domain by introducing a new
marker in the msm_iommu_domain for debug domains. This patch
has no functional impact.

Change-Id: I86dfbadab0a74f9018acc602eea28b2a3a2c38b5
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>