serial: 8250_pci: Add support for 16 port Exar boards

The Exar XR17V358 chip usually provides only 8 ports. But two chips can be
combined to act as a single 16 port chip. Therefor one chip is configured
as master the second as slave by connecting the mode pin to VCC (master)
or GND (slave).

Then the master chip is reporting a different device-id depending on
whether a slave is detected or not. The UARTs 8-15 are addressed from
0x2000-0x3fff. So the offset of 0x400 from UART to UART can be used to
address all 16 ports as before.

See: https://www.exar.com/common/content/document.ashx?id=1587 page 11

Signed-off-by: Soeren Grunewald <soeren.grunewald@desy.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>