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 <[email protected]>
Acked-by: Sascha Hauer <[email protected]>
Signed-off-by: Olof Johansson <[email protected]>
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