arm64: dts: renesas: r8a77990: Add display output support

The R8A77990 (E3) platform has one RGB output and two LVDS outputs
connected to the DU. Add the DT nodes for the DU, LVDS encoders and
supporting VSP and FCP.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>