drivers/serial: Add port 2 instance in ns16550 driver

Looks lik ARC arch snps_esmk can provide a 3rd port of this controller,
so let's add the necessary bits and pieces to get it instanciated if one
enable this port in DTS.

PCI settings are not introduced for that port as there is no known
arch/board exposing a 3rd port on PCI bus.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>