iommu: Make bus_iommu_probe() static

With the last external caller of bus_iommu_probe() now gone, make it
internal as it really should be.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Beleswar Padhi <b-padhi@ti.com>
Link: https://lore.kernel.org/r/a7511a034a27259aff4e14d80a861d3c40fbff1e.1730136799.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>