doc: usb: ci-hdrc-usb2: Add property "mux-controls"

The chipidea usb controller may be connected, in some platforms,
to an external mux to toggle between different usb ports for
different roles (host and device).

The mux-controller property, if set, binds the chipidea usb
controller with a mux for this use.

Signed-off-by: Yossi Mansharoff <yossim@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>