MAINTAINERS: Add myself as a bcm2835 co-maintainer.

The current maintainers have limited free time to work on the
architecture, and I'm motivated to do so for my work on graphics for
Broadcom.  Arnd and Florian suggested to me that this might be the way
forward.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>