drivers: serial: add support for uart_line_ctrl_set()

- For MCUX LPUART driver, added support to control RTS line
  High/Low from other driver/app code.

- This control is required to wakeup other device which is
  in sleep and configured its wakeup-source as UART-CTS line.

Signed-off-by: Nirav Agrawal <nirav.agrawal@nxp.com>