arm64: dts: r8a7796: salvator-x: Add DU external dot clocks

The DU1 external dot clock is provided by the fixed frequency clock
generator X21, while the DU0 and DU2 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>