serial: exar: Factor out platform hooks

This prepares the addition of IOT2040 platform support by preparing the
needed setup and rs485_config hooks.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>