driver: uart: ns16550: implement configure API

The UART configure API was added to uart.h and this commit
implements the initial framework for the configure API for
the ns16550 uart. This includes the configure() and config_get()
functions and uart device configuration structures of the uart
configure API for the ns16550.

Note this commit does not resolve the pre-existing hard-coded
8-n-1 implementation.

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