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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/writing-bindings.rst

···
   devices only need child nodes when the child nodes have their own DT
   resources. A single node can be multiple providers (e.g. clocks and resets).
 
+- DON'T treat device node names as a stable ABI, but instead use phandles or
+  compatibles to find sibling devices. Exception: sub-nodes of given device
+  could be treated as ABI, if explicitly documented in the bindings.
+
 - DON'T use 'syscon' alone without a specific compatible string. A 'syscon'
   hardware block should have a compatible string unique enough to infer the
   register layout of the entire block (at a minimum).
+
+- DON'T use 'simple-mfd' compatible for non-trivial devices, where children
+  depend on some resources from the parent. Similarly, 'simple-bus' should not
+  be used for complex buses and even 'regs' property means device is not
+  a simple bus.
 
 
 Properties