ARM: dts: r8a7779: marzen: Configure pinmuxing for the DU0 input clock

DU0 uses an externally provided clock, but the corresponding pin isn't
correctly muxed. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>