serial: 8250_exar: Move custom divisor support out from 8250_port

There are Exar custom divisor support in 8250_port which belongs to
8250_exar module. Move it out to the correct module and do not contaminate
generic code with it.

Cc: Aaron Sierra <asierra@xes-inc.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190731170558.52897-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>