arm64: dts: r8a7796: m3ulcb: Add DU external dot clocks

The DU0/DU1/DU2 external dot clocks are provided by the programmable
Versaclock5 clock generator.

Signed-off-by: Vladimir Barinov <vladimir.barinov+renesas@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>