ANDROID: KVM: arm64: iommu: Allow NULL domains in kvm_iommu_host_stage2_idmap

Some drivers might only have an identity page table that is not tracked
through the domain infrastructure.
Allow the domain to be passed as NULL in this case and it’s the driver
responsibility to do the right thing.

Bug: 277989609
Bug: 278749606
Bug: 346441377

Change-Id: Iceead524498da6ca5a4baa54d085ba2e8676871c
Signed-off-by: Mostafa Saleh <smostafa@google.com>