Remove powernv generic PCI controller operations. Replace it with
controller ops for each of the two supported PHBs.

As an added bonus, make the two new structs const, which will help
guard against bugs such as the one introduced in 65ebf4b6
("powerpc/powernv: Move controller ops from ppc_md to controller_ops")
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>