arm64: dts: renesas: r8a77990: ebisu: Add backlight

Add the backlight device for the LVDS1 output, in preparation for panel
support.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>