powerpc/8xx: Update email address in MAINTAINERS

Since 01 May 2020, our email adresses have changed to @csgroup.eu

Update MAINTAINERS accordingly.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9fd0f9a827ebbeae64ad7a6f6c595d242f4dd5fc.1588747860.git.christophe.leroy@csgroup.eu