ath79: drop and consolidate redundant chosen/bootargs

ath79: drop and consolidate redundant chosen/bootargs

In ath79, for several SoCs the console bootargs are defined to the
very same value in every device's DTS. Consolidate these definitions
in the SoC dtsi files and drop further redundant definitions elsewhere.

The only device without any bootargs set has been OpenMesh OM5P-AC V2.
This will now inherit the setting from qca955x.dtsi

Note that while this tidies up master a lot, it might develop into a
frequent pitfall for backports.

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