ARM: dts: alt: Correct clock of DU1

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

Fixes: 876e7fb9 ("ARM: shmobile: r8a7794: alt: Enable VGA port")
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>