ipq40xx: build DEVICE_DTS based on SOC and device name

ipq40xx: build DEVICE_DTS based on SOC and device name

This patch uses the SOC variable to calculate DTS names automatically
based on the SOC and the device definition node name.

This reduces redundancy and (by having to choose DTS name
appropriately) will unify the naming of a device in different places
(image/Makefile, DTS name, compatible, image name). This is supposed
to make life easier for developers and reviewers.

Since the kernel uses a "soc-device.dts" scheme for this target, we
use this for the derivation of DEVICE_DTS, too, and rename the files
not having followed it so far.

Note that for some devices the kernel itself is inconsistent, leaving
us with a manual overwrite for ap.dk01.1-c1 and ap.dk04.1-c1.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>