apalis_imx6: add device tree to makefile

apalis_imx6: add device tree to makefile

Add device tree to Makefile to avoid newly introduced error:

Device Tree Source is not correctly specified.
Please define 'CONFIG_DEFAULT_DEVICE_TREE'
or build with 'DEVICE_TREE=<device_tree>' argument

make[1]: *** [dts/Makefile:28: arch/arm/dts/imx6-apalis.dtb] Error 1
make: *** [Makefile:1009: dts/dt.dtb] Error 2

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>