MAINTAINERS: merge MXS entry into IMX one

The mach-mxs platform is actually co-maintained by myself and
pengutronix folks.  Also it's hosted in the same kernel tree as IMX.
So let's merge the entry into IMX one.

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