MAINTAINERS: Add maintainer entry for the MSM DRM driver

Rob and Archit are the main developers behind the driver.

v2: Removing Archit for now, correcting the status and adding
linux-arm-msm@ mailing list.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462230888-26319-1-git-send-email-emil.l.velikov@gmail.com