[LEDE-DEV] Convert ar71xx to devicetree

[p.wassi at gmx.at]

> Done. Pushed the cleanup commit to my staging tree

Thank you, Felix. This commit works perfectly on DT boards :)
(I did not test it on non-DT devices)

In the meantime I've prepared DT things here:
https://github.com/p-wassi/lede-source/tree/ath79_devicetree

This branch is compile-tested and run-tested for
TP-Link MR3020-v1 and WR740N-v4. Bootlogs are here:
https://pwassi.privatedns.org/lede/ath79-dt/

Best regards,
P. Wassi