MAINTAINERS: add entries for TCG

MAINTAINERS: add entries for TCG

The MAINTAINERS file was lacking entries concerning the TCG code, add
them based on the git history.

For the common TCG code, is probably better to keep qemu-devel@non-gnu.org
as this code can break easily, so it's better to get it reviewed by a few
persons.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>