powerpc/powernv: Change NPU PE# assignment

The P8+ hardware supports four partitionable endpoints (PEs) however
the hardware reports all errors as occurring on PE#0. This means we
need to reserve this PE for error handling (EEH) and not assign it to
a NPU device, implying that some devices will need to share PEs.

This patch changes the PE assignment for NPU devices such that NPU
devices which connect to the same GPU are assigned to the same
PE#.

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