Missing `reg` semantics for child nodes of indirect buses

[mbolivar-nordic]

As of 4fc81d4, the spec seems to
leave the semantics here undefined:

/ {
	bus {
		compatible = "indirect-bus";
		clock {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <32768>;
		};
	};
};

If we map in bus, do we always map in clock?

The current semantics for how nodes under an indirect-bus are included
or excluded rely on each node's reg property, which doesn't cover
this case.

[sstabellini]

You have a good point, we should clarify this. I think if bus is mapped, then also clock is mapped.

[mbolivar-nordic]

@sstabellini I thought about this more and I have another clarifying question: is this recursive?

For example, if clock has a child node without a reg property, is that also mapped?

[sstabellini]

Yes, also the child of clock should be mapped