Represent pins A6 and A7 of Arduino Nano similar to other pins about avr-hal

[Fundevoge]

I see, thanks for pointing it out.
It would be convenient to be able to use pins.a6 and then get a Pin<Analog, ADC6> or similar that does not implement the PinOps so it cannot be converted, but implements AdcChannel so a6.analog_read() is possible.
But I also understand that this is not the most important feature...

[Rahix]

They are supported, access just works a bit different than for all the other pins. Check the example code here:

avr-hal/examples/arduino-nano/src/bin/nano-adc.rs

Lines 47 to 53 in 06ad11f

	// Arduino Nano has two more ADC pins A6 and A7. Accessing them works a bit different from
	// the other pins as they are not normal IO pins. The code below shows how it works.
	let (a6, a7) = (
	adc.read_blocking(&adc::channel::ADC6),
	adc.read_blocking(&adc::channel::ADC7),
	);
	ufmt::uwrite!(&mut serial, "A6: {} A7: {}", a6, a7).void_unwrap();

[Rahix]

If you manage to make this work nicely, I'd be happy to take a PR implementing this :)

So far my reasoning was that writing code generic over ADC channels is already possible using AdcChannel so I didn't bother going any further here.