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>