powerpc: Update reviewers

Christophe and Nick have been active in recent years on the mailing list
and making contributions, add them as reviewers.

Paul and Ben are no longer actively reviewing powerpc patches, remove
them from the reviewers, they're still on linuxppc-dev if needed.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220629060817.2943966-1-mpe@ellerman.id.au