dt-bindings: example: Extend based on practice

dt-bindings: example: Extend based on practice

Extend the example schema with common rules which seems to be not that
obvious:
1. Expecting arrays of phandles to be always ordered, regardless if
   "xxx-names" is provided (e.g. clocks),
2. Add example of altering a property based on presence of other
   property,
3. Document usage of unevaluatedProperties.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200910184706.9677-1-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>