MAINTAINERS: add Mark Brown as a linux-next maintainer

Mark has been kindly helping fill in when I have been unavailable over
the past several years.  He has also put his hand up to take over
linux-next maintenance when I finally decide to stop (which may be some
time yet ;-) ).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>