ARM: dts: r8a7794: Correct clock of DU1

The second channel of the display unit uses a different module clock
than the first channel.

Fixes: 46c4f13d ("ARM: shmobile: r8a7794: Add DU node to device tree")
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>