iommu: msm: move msm_iommu_register_notify

It's currently inside an `#ifdef CONFIG_MSM_IOMMU_VBIF_CHECK', which is
incorrect since it doesn't actually depend on that config.  Drivers that
want to use it fail to link when CONFIG_MSM_IOMMU_V1=y &&
CONFIG_MSM_IOMMU_VBIF_CHECK=n.  Move it out and export it while we're at
it.

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Change-Id: I48dc655fbd1558871704aa40e065e1002e836308