usb: musb: re-introduce musb->port_mode

Define MUSB_PORT_MODE_{HOST,GADGET,DUAL_ROLE} and store the
platform-specified value in struct musb.

Note that MUSB_PORT_MODE_HOST has to be set to 1 in order to match
existing device tree bindings which are already documented but in fact
unusued. For information on the bindings, please refer to

  Documentation/devicetree/bindings/usb/omap-usb.txt
  Documentation/devicetree/bindings/usb/am33xx-usb.txt

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>