serial: sh: Add support External Clock mode

R8A7780 and R7A7791 of rmobile supports External Clock mode, and these uses
different from Internal Clock mode registers and calculations to the baud rate
setting. This adds function for External Clock mode.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>