piscreen-overlay: Add invx, invy and swapxy

[satmandu]

This PR adds touchscreen x,y swap and inversion options, using the invx, invy, and swapxy as per the discussion at #3263 , which will ideally work for the Waveshare 3.5a touchscreen.

I have changed the parameter names to match the common parameter name usage in other device tree overlay files in the same dir.

[satmandu]

The dtbo appears to work for me. For whatever reason with the way the screen is oriented on the wavshare35a, I had to use the invx parameter to get the invy behavior I wanted (the touchscreen responding as expected to roughly where I poke at the screen.)

Is this because of the default swapxy?

Should I flip things so the settings are thus @6by9 ?

		invx =		<&piscreen_ts>,"touchscreen-inverted-y?";
		invy =		<&piscreen_ts>,"touchscreen-inverted-x?";

[6by9]

Is this because of the default swapxy?

Almost certainly.
With swapxy and invx/invy it matters which order the two operations are done in. It should be documented (somewhere) which order the two operations are done, and that is the expected behaviour.

Should I flip things so the settings are thus @6by9 ?

		invx =		<&piscreen_ts>,"touchscreen-inverted-y?";
		invy =		<&piscreen_ts>,"touchscreen-inverted-x?";

No, because then using invx without swapxy would give the wrong result.

[satmandu]

Happy to make documentation changes here as you might suggest.

Otherwise I'm happy to leave this as is, as this completes the functionality I desire.

[6by9]

It is already documented - https://github.com/raspberrypi/linux/blob/rpi-6.6.y/Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml#L73-L76

  touchscreen-swapped-x-y:
    description: X and Y axis are swapped
                 Swapping is done after inverting the axis
    type: boolean

I'm happy.
I'll leave it with @pelwell to make any further comments and merge.