dt-bindings: example-schema: don't use enum as fallback, explain clock-names

dt-bindings: example-schema: don't use enum as fallback, explain clock-names

Compatibles with multiple entries should have usually only one fallback
compatible thus enum followed by enum is not a common case.  Use 'const'
as second compatible to show the recommended approach.

Explain also when clock-names are not really necessary.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230612092611.12385-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>