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

While this is a cosmetic change, backporting it to 19.07 will be a
major help for anyone doing backports of device support. Without it,
every backporter would have to remember to manually add the chosen node
to the device's DTS.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 635f111148c3f7ccb0ecc92863a3b1a142f6ebeb)