uart/ns16550: implements new API of line control

This implements the newly introduced APIs to manipulate line controls
for UART.

This also implements the driver command API as setting baud rate
may require setting the DLF.

Change-Id: I80634aa3fbd4a3223c46edb61417123e36f439dd
Signed-off-by: Daniel Leung <daniel.leung@intel.com>