MAINTAINERS: update avr32 web ressources

Web resource http://avr32linux.org/ is no longer available.  We add the
mirror of the web page foud at http://mirror.egtvedt.no/avr32linux.org/

.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>