ath79: create shared DTSI/definition for Belkin F9J1108/F9K1115 v2

ath79: create shared DTSI/definition for Belkin F9J1108/F9K1115 v2

Belkin F9J1108 v2 and F9K1115 v2 are (seemingly) identical hardware
with different model numbers. Extract all non-device specific code to a
common .dtsi so it can be re-used when adding support for the
F9K1115 v2.

Similar to the .dtsi most of the image building recipe code can be
re-used. Move everything except the device model, edimax header magic
and edimax header model into a shared build recipe.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
[drop duplicate TARGET_DEVICES, add EDIMAX_* to DEVICE_VARS, edit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>