MAINTAINERS: new git entry for arm/mach-msm

The msm git tree moved to

  git://git.kernel.org/pub/scm/linux/kernel/git/davidb/linux-msm.git

Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>