MAINTAINERS: add ARM/FREESCALE MXS entry

As suggested by Pengutronix, they want to get Sascha relieved from
the burden of maintaining MXS sub-architecture.  Since I brought the
most of MXS core code to mainline (with the great help from
Pengutronix people, thanks!), I would like to step up for maintaining
MXS.

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