ath79: restore support for boot console with arbitrary baud rates

ath79: restore support for boot console with arbitrary baud rates

The Arduino Yun uses a baud rate of 250000 by default. The serial is
going over the Atmel ATmega and is used to connect to this chip.
Without this patch Linux wants to switch the console to 9600 Baud.

With this patch Linux will use the configured baud rate and not a
default one specified in uart_register_driver().

This has been added for ath79 4.19 and 5.4 in master as part of
fc59b2f79b50 ("ath79: add support for Arduino Yun"), this backports
it separately to 4.14.

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