devicetree: support generating symbols for -gpios properties w/o compatible

[pabigot]

A recent change to devicetree tooling allowed generation of nodes for which there was no compatible, but that generation did not include any properties on such nodes (which I still think is weird).

Do we have enough special-casing in the tooling to support something like:

/ {
  app {
    power-gpios = <&gpioa 5 GPIO_ACTIVE_HIGH>;
  };
};

then look it up by DT_PATH(app, power_gpios)? At least for GPIO we can reasonably say that if the property ends in -gpios we can assume it's a GPIO phandle+specifier. @galak?

Originally posted by @pabigot in #25596

[pabigot]

From #24769 (review) re converting other properties where the corresponding representation can be inferred w/o recourse to a binding would be useful:
I added these to /:

       special = "this";
       subspecial {
          bar = "that";
       };

The subordinate node was generated, but neither /special nor /subspecial/bar were generated

Is that what was intended? Generating the node without its properties seems odd; will this satisfy the need identified in #24460 (comment)

[pabigot]

Also needed for things like:

servo-pwms = <&pwm0 17>;

per a slack discussion.

[sslupsky]

Yeah, I ran into this issue a while ago too. This would be useful.

[pabigot]

This keeps coming up. @galak Do you have fundamental objections to this that will prevent us from implementing it, or can we proceed?

@mbolivar-nordic It would probably be more efficient for you to do this, but if you're fully booked I'm willing to take it on.

[mbolivar-nordic]

I don't think I'll have cycles for this anytime soon

[carlescufi]

API meeting 4th August 2020

• The functionality in itself is welcome by most
• The namespace is however under debate:
  • Arbitrary namespace (app. mygpios, etc): not gathering any support
  • Fixed namespace (zephyr,user). The tooling can then limit the generation of these (nodes without a compatible) when they are under the fixed namespace: /zephyr,user/<user-chosen-name>-{gpios, pwms, ..}

{
  zephyr,user {
    power-gpios = <&gpioa 5 GPIO_ACTIVE_HIGH>;
  };
}

• The use of these nodes should be limited to samples/tests but not drivers
• The use of chosen for this does not seem to match the original intent of chosen in Zephyr (limited set of known chosen nodes)

[carlescufi]

This proposal would effectively replace the ones in #18544 and #15020