[POWERPC] iseries: Move iommu_table_cb into platforms/iseries

Although we pass the address of an iommu_table_cb to HvCallXm_getTceTableParms,
we don't actually need the structure definition anywhere except in the
iseries iommu code, so move the struct in there.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>