misc: apq8084_dock: Add/remove USB when docking

Add a device tree handle to the USB controller that handles the
docking port. This is to allow the USB device driver to be removed
whenever the device is undocked, allowing for increased power savings.
Likewise, when the dock is attached, reinstantiate the USB driver.

Change-Id: I40e23c75ae4acf02fdbb14fd0e86cce6957e181b
Signed-off-by: Jack Pham <jackp@codeaurora.org>