powerpc/powernv: Use OPAL call for TCE kill on NVLink2

Add detection of NPU2 PHBs. NPU2/NVLink2 has a different register
layout for the TCE kill register therefore TCE invalidation should be
done via the OPAL call rather than using the register directly as it
is for PHB3 and NVLink1. This changes TCE invalidation to use the OPAL
call in the case of a NPU2 PHB model.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>