arm64: dts: ulcb: Add DU external dot clock sources

The DU0/DU1/DU2/DU3 external dot clocks are generated by an I2C-controlled
programmable clock generator.

Clock generator is available on both the H3 and M3-W ULCB boards.
Add this to the ulcb.dtsi file.

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>