ipq806x: build DEVICE_DTS based on SOC and device name

ipq806x: 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, leaving the DTS names
unchanged for this target.

Note that for some devices the kernel itself uses inconsistent names
(DTS naming scheme vs. compatible), leaving us with a manual overwrite
of DEVICE_DTS for those cases.

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