iommu: arm-smmu: Fix dynamic domain context bank assignment

Fix an issue where the context bank for a dynamic domain is always set
to 0xFF. The expected call flow is:
0) iommu_domain_alloc()
1) iommu_domain_set_attr(DOMAIN_ATTR_DYNAMIC)
2) iommu_domain_set_attr(DOMAIN_ATTR_CONTEXT_BANK)
3) iommu_attach_device/group()

Change-Id: I134c2fdbea9d9efc7306d7dbe03abb5dae431371
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>