uart/pci: support 16550A PCI serial devices

Expand the current check to also attach the ns8250 driver to devices
reporting as 16550A. This has been tested to work on a real device.

From an inspection of the code in the ns8250 driver it seems like it should
support up to 16950A devices, but I don't have hardware to ensure that,
hence be conservative with the change.

MFC: 2 weeks
Reviewed by: imp
Differential revision: https://reviews.freebsd.org/D56095