usb: of: add functions to bind a companion controller

EHCI controllers will have a companion controller. However, on platform
bus, there was difficult to bind them in previous code. So, this
patch adds helper functions to bind them using a "companion" property.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>