MAINTAINERS: merge IMX6 entry into IMX

I have been co-maintaining IMX sub-architecture for a couple of years,
and collecting IMX sub-architecture patches rather than IMX6 only ones
for a few release cycles.  It makes sense to officially add myself as
the co-maintainer for IMX sub-architecture now.  Consequently, IMX6
entry can just be merged into IMX.

While at it, add a 'F:' entry for IMX DTS files.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Olof Johansson <olof@lixom.net>