ath79: DTS file style update and harmonization

ath79: DTS file style update and harmonization

This applies several style adjustments that have been requested in
recent reviews to older DTS files. Despite making the code base more
consistent, this will also help to reduce review time when DTSes
are copy/pasted.

Applied changes:
- Rename gpio-keys/gpio-leds to keys/leds
- Remove node labels that are not used
- Use label property for partitions
- Prefix led node labels with "led_"
- Remove redundant includes
- Harmonize new lines after status property
- Several smaller style fixes

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