spapr_iommu: Make spapr_tce_find_by_liobn() public

spapr_iommu: Make spapr_tce_find_by_liobn() public

At the moment spapr_tce_find_by_liobn() is used by H_PUT_TCE/...
handlers to find an IOMMU by LIOBN.

We are going to implement Dynamic DMA windows (DDW), new code
will go to a new file and we will use spapr_tce_find_by_liobn()
there too so let's make it public.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>