"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/sh73a0-kzm9g.dt.yaml: extcki: 'clock-frequency' is a required property

Fix this by adding the missing "clock-frequency" properties to the various
clocks, to be overridden by the board DTS files when populated.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191213162736.2160-1-geert+renesas@glider.be