ARM: dts: imx6-tx6: remove obsolete ipu1 alias

The 'ipu1' alias is already defined in imx6q.dtsi. There is no need to
redefine it here.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>