cc2538: usb: Make the GPIO driving the pull-up optional
cc2538: usb: Make the GPIO driving the pull-up optional

The data sheet recommends that the USB pull-up resistor be driven by a GPIO so
that it can be controlled by software, but this is not mandatory. Hence, leave
the choice so that CC253-based boards not using this option can build and work
fine.

Signed-off-by: Benoît Thébaudeau <835bc8b01b82c0117127b72626b289641d7633e2@advansee.com>

MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,bluerover/6lbr,bluerover/6lbr