ARM: dts: am3874-iceboard: Fix 'i2c-mux-idle-disconnect' usage

According to
Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.txt,
i2c-mux-idle-disconnect is a property of a parent node since it
pertains to the mux/switch as a whole, so move it there and drop all
of the concurrences in child nodes.

Fixes: d0317731 ("ARM: dts: Adds device tree file for McGill's IceBoard, based on TI AM3874")
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Benoît Cousson <bcousson@baylibre.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Graeme Smecher <gsmecher@threespeedlogic.com>
Cc: linux-omap@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Tested-by: Graeme Smecher <gsmecher@threespeedlogic.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>