MAINTAINERS: match freescale ARM64 DT directory in i.MX entry

The majority of device trees in arch/arm64/boot/dts/freescale/ are
built around i.MX SoCs with the rest being for Layerscape. Yet, calling
get_maintainers.pl -f on this directory will not match the MAINTAINERS
entry, because the directory name doesn't contain the substring "imx".
Add an explicit file match for the directory and exclude the Layerscape
specific files. This ensures To/Cc is not only generated from git
history, but takes e.g. the R: entries into account as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>