MAINTAINERS: Update PPC status and maintainer

Richard agreed to make odd fixes to PPC tcg parts[1]. This patch makes
the change.

[1] https://lists.gnu.org/archive/html/qemu-ppc/2016-03/msg00657.html



Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>