arm64: dts: renesas: salvator-x: Add DU external dot clock sources

The DU1 and DU2 external dot clocks are fixed frequency clock generators
running at 33MHz, while the DU0 and DU3 external dot clocks are
generated by an I2C-controlled programmable clock generator.

All those clock generators are available on both the H3 and M3-W
Salvator-X boards. Add them to the salvator-x.dtsi file.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>