parl_io uses InputConnection instead of PeripheralInput

[bjoernQ]

see #2492 (comment)

If a driver requires pins, those pins must be configured using

fn with_signal_name(self, pin: impl Peripheral<P = InputConnection> + 'd) -> Self)
or
fn with_signal_name(self, pin: impl Peripheral<P = OutputConnection> + 'd) -> Self)

We currently only do that for parl_io and the doc for InputConnection say:

This is mainly intended for internal use, but it can be used to connect peripherals within the MCU without external hardware.

Given the comment I'd say let's not mark this (InputConnection) as stable to reduce the stable API surface but the advice in the API guidelines suggest we shouldn't - however there is a mismatch between the guidelines and the API docs (and the reality of existing drivers). Similarly OutputConnection (Bjoern).

[bugadani]

I'm trying to find where exactly this is a problem, but all the public APIs I see use PeripheralInput and PeripheralOutput. Internally these are converted into Input/OutputConnection but that should be fine.