dt-bindings: mfd: syscon: Split and enforce documenting MFD children

Simple syscon nodes can be documented in common syscon.yaml, however
devices with simple-mfd compatible, thus with some children, should have
their own schema listing these children.  Such listing makes the binding
specific, allows better validation (so the incorrect child would not
appear in the simple-mfd node) and actually enforces repeated rule for
simple-mfd devices:

  "simple-mfd" is only for simple devices, where the children do not
  depend on the parent.

Currently the syscon+simple-mfd binding is quite broad and allows
any child or property, thus above rule cannot be enforced.

Split the syscon.yaml binding into:
1. Common syscon properties, used potentially by many bindings.
2. Simple syscon devices (NO simple-mfd!).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240627-dt-bindings-mfd-syscon-split-v4-6-dc6699a9f3e4@linaro.org


Signed-off-by: Lee Jones <lee@kernel.org>