drivers: uart: ns16550: move from hard-coded to configurable

The NS16550 UART driver is currently hard-coded as 8-n-1
with no flow control. The baud rate is set by what is in DTS.
This commit moves away from hard-coded and strictly DTS to
configurable using the UART configure API. Requires commit #bcb807.

Signed-off-by: Jennifer Williams <jennifer.m.williams@intel.com>