MAINTAINERS: Explicitly add linux-arm-kernel for bcm2835

get_maintainers.pl would list linux-arm-kernel for most bcm2835
changes because it's under arch/arm, but might not for other bcm2835
drivers.  Most ARM architectures appear to list linux-arm-kernel as an
appropriate list as well.

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