scripts: dts: Change 'child/parent: bus: ...' to 'child/parent-bus:'

Instead of

    child:
        bus: foo

    parent:
        bus: bar

, have

    child-bus: foo

    parent-bus: bar

'bus' is the only key that ever appears under 'child' and 'parent'.

Support the old keys for backwards compatibility, with a deprecation
warning if they're used.

Also add 'child/parent-bus' tests to the edtlib test suite. It was
untested before.

I also considered putting more stuff under 'child' and 'parent', but
there's not much point when there's just a few keys I think. Top-level
stuff is cleaner and easier to read.

I'm planning to add a 'child-binding' key a bit later (like 'sub-node',
but more flexible), and child-* is consistent with that.

Also add an unrelated test-bindings/grandchild-3.yaml that was
accidentally left out earlier.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>