ASoC: dt-bindings: amlogic-sound-cards: document

Merge series from Neil Armstrong <neil.armstrong@linaro.org>:

Following an off-list discution with Jerome about fixing the following
DTBs check errors:
    sound: Unevaluated properties are not allowed ('assigned-clock-parents', 'assigned-clock-rates', 'assigned-clocks' were unexpected)
        from schema $id: http://devicetree.org/schemas/sound/amlogic,axg-sound-card.yaml#
    sound: Unevaluated properties are not allowed ('assigned-clock-parents', 'assigned-clock-rates', 'assigned-clocks' were unexpected)
        from schema $id: http://devicetree.org/schemas/sound/amlogic,gx-sound-card.yaml#
    sound: 'anyOf' conditional failed, one must be fixed:
        'clocks' is a required property
        '#clock-cells' is a required property
        from schema $id: http://devicetree.org/schemas/clock/clock.yaml#

It has been agreed documenting the clock in the sound card is a better solution
than moving them to a random clock controller or consumer node which is not
related to the actual meaning of those root frequencies.

The patchset adds the clocks proprty to the bindings and finally adds the
properties to the DT files.