8250_pci: add support for National Instruments legacy 8420 RS232 boards

Signed-off-by: Will Page <will.page@ni.com>
Signed-off-by: Shawn Bohrer <shawn.bohrer@ni.com>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>