powerpc: Switch 8xx MAINTAINERS entry to Christophe

It's over 10 years since the last commit from Vitaly, so I suspect
he's moved on to other things.

Christophe has been the primary contributor to 8xx in the last several
years, so anoint him as the maintainer.

Remove the dead penguingppc.org link.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Christophe Leroy <christophe.leroy@c-s.fr>
Link: https://lore.kernel.org/r/20200225092534.9587-1-mpe@ellerman.id.au