powerpc/iommu: Move dev_has_iommu_table() to iommu.c

Move function dev_has_iommu_table() to powerpc/kernel/iommu.c
as it is going to be used by machine specific iommu code as
well in subsequent patches.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/171923274748.1397.6274953248403106679.stgit@linux.ibm.com