ARM: shmobile: r8a7791: koelsch: Fix pinmux for HDMI

The signals output by the DU to the HDMI transmitter use full 24-bit
RGB. Make sure all pins are properly muxed.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7791-koelsch.dts

@@ -326,7 +326,7 @@ i2c2_pins: i2c2 {
 	};
 
 	du_pins: du {
-		renesas,groups = "du_rgb666", "du_sync", "du_disp", "du_clk_out_0";
+		renesas,groups = "du_rgb888", "du_sync", "du_disp", "du_clk_out_0";
 		renesas,function = "du";
 	};