arm64: dts: renesas: r8a77965: Populate the DU instance placeholder

The DU entity node has been previously added but only as a placeholder.
Populate the node with the properties to use the device.

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>