serial: 8250_pci: add support for the AX99100

This is found popular brands such as StarTech.com or Delock, and has
been a source of frustration to quite a few people, if I can trust
Amazon comments complaining about Linux support via the official
out-of-the-tree driver.

Signed-off-by: Martin Peres <martin.peres@mupuf.org>