MAINTAINERS: TURBOchannel: Update entry

Add mailing list and patchwork URL so patches now have a defined path.
Also add myself as a TURBOchannel maintainer.  However Maciej is going
to the the resident TURBOchannel guru.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: "Maciej W. Rozycki" <macro@linux-mips.org>