docs: dt: writing-bindings: Document node name ABI and simple-mfd

Document established Devicetree bindings maintainers review practice:

1. Device node names should not be treated as an ABI, unless for
   children of a device when documented.
   There were many patches posted using of_find_node_by_name() or
   of_node_name_eq() for accessing siblings or completely different
   nodes.  These cases were introducing undocumented ABI, so they are
   discouraged.

2. 'simple-mfd' means children do not depend on parent device resources.
   'simple-bus' is so simple, that even 'reg' properties are not
   applicable.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250818132534.120217-2-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>