dt-bindings: clock: add meson axg audio clock controller bindings

Export the clock ids dt-bindings usable by the consumers of the clock
controller and add the documentation for the device tree bindings of
the audio clock controller of the A113 based SoCs.

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>