powerpc/powernv: Discover IODA3 PHBs

We instanciate them as IODA2. We also change the MSI EOI hack
to only kick on PHB3 since it will not be needed on any new
implementation.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>