[DCCP]: Add MAINTAINERS and CREDITS entries

Also remove the SPX entry in MAINTAINERS, forgot to do that
when I removed it.

Signed-off-by: Ian McDonald <iam4@cs.waikato.ac.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>