arm64: dts: r8a7795: Add support for the DU

Add a compatible string and VSP links to the DU node. The H3 ES1.x and
H3 ES2.0 are compatible save for the links to the VSPs that are
described explicitly in DT, so there's no need for a new ES2-specific
compatible string.

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>