serial: exar: split out the exar code from 8250_pci

Add the serial driver for the Exar chips. And also register the
platform device for the GPIO provided by the Exar chips.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>