ARM: dts: imx6qdl: add multiplexer controls

The IOMUXC General Purpose Register space contains various bitfields
that control video bus multiplexers. Describe them using a mmio-mux
node. The placement of the IPU CSI video mux controls differs between
i.MX6D/Q and i.MX6S/DL.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>