iommu/arm-smmu: Assign page table memory on domain finalization

When a domain is attached for the first time, memory for its top level
of page tables is allocated.  If it's a secure domain, this memory needs
to be assigned to a secure VMID.  Currently, it's not being assigned to
a secure VMID, so when the domain is torn down and unassigned we get a
warning.  Fix this by adding the missing assign call after initial
domain allocation.

Change-Id: I9ff2418749e768e17b4598f5f89e587ad7828d52
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>