ramips: move redundant console setup to mt7621 SoC DTSI

For mt7621, console is set up via DTS bootargs individually in
device DTS/DTSI files. However, 44 of 74 statements use the
following setting:

	chosen {
		bootargs = "console=ttyS0,57600";
	};

Therefore, don't repeat ourselves and move that definition to the SoC
DTSI file to serve as a default value.

This patch is cosmetic.

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