mxs: rename files to files-4.9

The Device Tree file for I2SE Duckbill boards was updated upstream.
Let's use the upstream version for upcoming kernel 4.14 by keeping
our current version for v4.9 still around.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

diff --git a/target/linux/mxs/files/arch/arm/boot/dts/imx28-duckbill.dts b/target/linux/mxs/files-4.9/arch/arm/boot/dts/imx28-duckbill.dts
similarity index 100%
rename from target/linux/mxs/files/arch/arm/boot/dts/imx28-duckbill.dts
rename to target/linux/mxs/files-4.9/arch/arm/boot/dts/imx28-duckbill.dts