Update powerpc maintainers

Ben Herrenschmidt is taking over as the primary powerpc architecture
maintainer.  I'll still be around as his backup/deputy.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>