arm64: dts: renesas: r8a7795-salvator-x: Add DU external dot clocks

The DU1 and DU2 external dot clocks are provided by the fixed frequency
clock generators X21 and X22, while the DU0 and DU3 clocks are provided
by the programmable Versaclock5 clock generator.

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>