ar71xx: restore support for boot console with arbitrary baud rates

ar71xx: restore support for boot console with arbitrary baud rates

Commit 1bfbf2de6df9 ("ar71xx: serial: core: add support for boot console
with arbitrary baud rates") added support for arbitrary baud rates which
enabled 250000 baud rate for Yun. But the patch was not ported to kernel
4.9, and since then the kernel set its baud rate to 9600. This commit ports
the patch to kernel 4.14, thereby restoring the serial console of Yun.

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit c90db26e051e4f0e7cd32333b3bd8c94a13d599a)