arm64: dts: renesas: beacon kit: Setup AVB refclk

The AVB reference clock assumes an external clock that runs
automatically.  Because the Versaclock is wired to provide the
AVB refclock, the device tree needs to reference it in order for the
driver to start the clock.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20210224115146.9131-5-aford173@gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>