ARM: dts: imx6qdl: move iomuxc compatible assignment out of root node

The common imx6qdl.dtsi already defines the iomuxc phandle. Make use of
it in the imx6dl.dtsi and imx6q.dtsi.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>