iommu: msm: add msm_iommu_register_notify stub for CONFIG_MSM_IOMMU_V1=n

Drivers using msm_iommu_register_notify currently don't link when
CONFIG_MSM_IOMMU_V1=n.  We may want to implement the notifier for
msm_iommu_v0 in the future, but until then provide a stub to allow
successful compilation and linking of driver using
msm_iommu_register_notify.

Change-Id: I7c8c91ed2e08a4eb473f0b6e3906e9e6d473f1ca
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>