serial: exar: Add support for IOT2040 device

This implements the setup of RS232 and the switch-over to RS485 or RS422
for the Siemens IOT2040. That uses an EXAR XR17V352 with external logic
to switch between the different modes. The external logic is controlled
via MPIO pins of the EXAR controller.

Only pin 10 can be exported as GPIO on the IOT2040. It is connected to
an LED.

As the XR17V352 used on the IOT2040 is not equipped with an external
EEPROM, it cannot present itself as IOT2040-variant via subvendor/
subdevice IDs. Thus, we have to check via DMI for the target platform.

Co-developed with Sascha Weisenberger.

Signed-off-by: Sascha Weisenberger <sascha.weisenberger@siemens.com>
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>