ARM: dts: imx6q: clean up unused ipu2grp

The pinctrl group ipu2grp is a leftover from the previous iomuxc DT
cleanup.  It's not used by anyone now.  More importantly, it's getting
in the way of saving the unnecessary pinfunc container node from the
board dts files that include imx6q.dtsi.

Let's clean it up.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Tested-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Olof Johansson <olof@lixom.net>