docs: dt: writing-schema: Describe defining properties in top-level

docs: dt: writing-schema: Describe defining properties in top-level

Document established Devicetree bindings maintainers review practice:
Properties having differences per each device in the binding, e.g.
different constraints for lists or different allowed values, should
still be defined in top-level 'properties' section and only customized
in 'if:then:'.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250904142400.179955-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>