update MAINTAINERS file

update MAINTAINERS file

There have been some "organizational changes" within AMD.
Update the MAINTAINERS file accordingly.
Please also feel free to use the new email addresses if there are
any inquires related to code authored by Wei Wang, Christoph Egger
or Andre Przywara.

Signed-off-by: Andre Przywara <osp@andrep.de>
Acked-by: Wei Wang <weiwang.dd@gmail.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
Acked-by: Wei Huang <wei.huang2@amd.com>
Acked-by: Christoph Egger <Christoph_Egger@gmx.de>
Committed-by: Keir Fraser <keir@xen.org>