UART flow control

* RTS(Request to Send) flow control

nRTS is asserted (tied low) as long as the USART receiver is ready to receive new data. 

When the receive register is full, nRTS is deasserted.

* CTS(Clear to Send) flow control

The transmitter checks the nCTS input before transmitting the next frame.

If nCTS is asserted (tied low), then the next data is transmitted else the transmission does not occur.