powerpc/powernv: Reserve PE#0 on NPU

P8+ hardware reports all errors on PE#0. This patch ensures PE#0 is
not assigned to NPU devices so that it can be used for EEH.

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