MAINTAINERS: mark arch/blackfin/ and its gubbins as orphaned

The blackfin architecture has seen no maintainer action of any kind since
April 2015.  No new code, no pull requests, no acks to patches, no response
to mails, nothing.

The web site has an expired certificate (expiration Sep 2017, issued in
2013), the mailing list sees no answers either, with one exception:

  https://sourceforge.net/p/adi-buildroot/mailman/adi-buildroot-devel/


  >
  > Steven is no longer working on this for ADI. Acked by me if this works. Thanks.
  >
  > Best regards,
  > Aaron Wu
  > Analog Devices Inc.

But, Aaron doesn't seem to respond to queries either.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>