dt-bindings: usb: add the documentation for USB HCDs

A USB HCD may have several PHYs which need to be configured before the
the HCD starts working.
This adds the documentation for such a USB HCD as well as a reference to
the new "usb-hcd.txt" from all bindings that implement a USB HCD which
support one USB PHY per port.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Yixun Lan <yixun.lan@amlogic.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.con>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>