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: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: NIgor Opaniuk <igor.opaniuk@toradex.com>

arch/arm/dts/Makefile

@@ -591,6 +591,7 @@ dtb-$(CONFIG_MX6ULL) += \
 	imx6ull-dart-6ul.dtb
 
 dtb-$(CONFIG_ARCH_MX6) += \
+	imx6-apalis.dtb \
 	imx6-colibri.dtb
 
 dtb-$(CONFIG_MX7) += imx7d-sdb.dtb \