MAINTAINERS: powerpc: Transfer PPC83XX to Christophe

Christophe volunteered[1] to maintain PPC83XX.

1: https://lore.kernel.org/all/7b1bf4dc-d09d-35b8-f4df-16bf00429b6d@csgroup.eu/



Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Crystal Wood <oss@buserror.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231205051239.737384-1-mpe@ellerman.id.au