arm64: dts: renesas: r8a77965-salvator-xs: Enable DU external clocks and HDMI

The DU1 external dot clock is provided by the fixed frequency clock
generator X21, while the DU0 and DU3 clocks are provided by the
programmable Versaclock6 clock generator.

Enable the clocks, and the HDMI encoder for the M3-N Salvator-XS, and
hook it up to the HDMI connector

Based on patches from Takeshi Kihara <takeshi.kihara.df@renesas.com>

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>