MAINTAINERS: remove arch/arm/mach-mx*/ from IMX entry

With commit 784a90c0 ("ARM i.MX: Merge i.MX5 support into
mach-imx") merging mach-mx5 into mach-imx, arch/arm/mach-mx* only
covers mach-mxs now.  Since we already have a separate MXS entry
for arch/arm/mach-mxs, arch/arm/mach-mx* can simply be removed there.

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