ath79: create common DTSI for Senao ar934x APs

This creates a shared DTSI for ar934x Senao/Engenius APs:

 - EAP300 v2
 - ENS202EXT v1
 - EAP600
 - ECB600

Since ar9341/ar9344 have different configuration, this new file
mostly contains the partitioning.

Suggested-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>