serial: 8250: add driver for NI UARTs

The National Instruments (NI) 16550 is a 16550-like UART with larger
FIFOs and embedded RS-232/RS-485 transceiver control circuitry. This
patch adds a driver that can operate this UART, which is used for
onboard serial ports in several NI embedded controller designs.

Portions of this driver were originally written by Jaeden Amero and
Karthik Manamcheri, with extensive cleanups and refactors since by
Brenda Streiff.

Cc: Gratian Crisan <gratian.crisan@emerson.com>
Co-developed-by: Jason Smith <jason.smith@emerson.com>
Signed-off-by: Jason Smith <jason.smith@emerson.com>
Signed-off-by: Chaitanya Vadrevu <chaitanya.vadrevu@emerson.com>
Link: https://lore.kernel.org/r/a3b0df6d-1dd5-4cc4-a7e1-4ed51fb9e4cc@emerson.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>