The current atmel-samd port does not support RTS/CTS for hardware flow control, even though the underlying SAMx2x/SAMx5x chips are capable of using this mode on async UARTs. This change enables hardware flow control by allowing assignment of pins to RTS/CTS, and enabling selection of the correct pad configuration for the pinout via the CTRLA.TXPO register.

An example of working code looks like this (using the pin configs on the Sparkfun SAMD51 MicroMod):

import board
import busio

# On the SAMD51 MicroMod, SERCOM5 is used for UART1, and the pins that would be assigned to RTS/CTS are PB00/PB001
uart = busio.UART(board.UART_TX1, board.UART_RX1, cts=board.PWM0, rts=board.A1)
uart.write(bytearray("hello")) # Write 'hello' to the UART
uart.readline() # gets whatever is being written from the other side

I have tested this with on the MicroMod, and it works as expected when the other side is set up for hardware flow control. To fully confirm it, I disabled hardware flow control in picocom, attached to an FTDI USB-to-serial board wired to the UART RX/TX/RTS/CTS lines on the MicroMod, then used picocom's toggle features to assert RTS and CTS to prevent reads or writes as desired. I also confirmed that normal operation of the UART when hardware flow control is not specified works as it did prior to this change.

A note re confirming this PR: while RTS/CTS flow control is available on the SAMx2x/SAMx5x chips, it's only happenstance that a given dev board exposes the correct pins used for the given SERCOM underlying the UART(s) exposed via hardware. I found that several boards did not have the pins I need even connected. The MicroMod was one where I was able to find the right pads connected to pins.