MAINTAINERS: Remove linux-mips.org references

The linux-mips.org site has gone down and no replacement is available at
the moment.  Remove/update references in MAINTAINERS accordingly.  There
are a bunch of Kconfig references still present; keep them around for a
possible future update or for people to refer to via archive.org.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>