Move i2cmux node from soc node to root node.

i2cmux node does not have any register properties and thus shouldn't be
placed on the bus.

This fixes the following build warnings with W=1:

arch/arm/boot/dts/imx28-cfa10049.dtb: Warning (simple_bus_reg): Node /apb@80000000/apbx@80040000/i2cmux missing or empty reg/ranges property
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>