"clock-frequency" is a required property for devices nodes compatible
with "fixed-clock", leading to warnings when running

    $ make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/clock/fixed-clock.yaml
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_a: 'clock-frequency' is a required property
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_b: 'clock-frequency' is a required property
    arch/arm/boot/dts/r8a7778-bockw.dt.yaml: audio_clk_c: 'clock-frequency' is a required property

Fix this by adding the missing "clock-frequency" properties to the audio
clocks, to be overridden by board DTS files when populated.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191213162712.2056-1-geert+renesas@glider.be