ARM: dts: imx6: fix IPU1 DI1 node name

This node describes the DI1 port of IPU, fix the node name to reflect this.
There's currently no user of this node in mainline, so this change should
not break any supported platforms.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>