apm821xx: force WNDR4700 console to 115200n8

apm821xx: force WNDR4700 console to 115200n8

With the upcoming 4.19 release, the serial console on the
Netgear WNDR4700 would no longer work as it is never really
set and always relied on the kernel's serial code not to
change the baud rate.

On the stock firmware, Netgear forced the console setting
through a custom CONFIG_CMDLINE in their kernel to 115200.
Normally, they should have done it in a different way and
just passed the baudrate through a "console=..." kernel
parameter via the bootargs in the "/chosen" dt node.
But in their default u-boot bootcmd setting, they somehow
forgot to add the "run addtty", so there's no easy way to
pass the baudrate from u-boot to the kernel. So it has
to be forced as otherwise it ends up as 9600 baud.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>