ARM: dts: r8a7794: Add DU1 clock to device tree

Add the missing module clock for the second channel of the display unit.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>