usb: gadget: dwc2_udc_otg: implement pullup()

Pullup is used by the usb framework in order to do software-controlled
usb_gadget_connect() and usb_gadget_disconnect().

Implement pullup() for dwc2 using the SOFT_DISCONNECT bit in the dctl
register:
* when pullup is on, clear SOFT_DISCONNECT
* when pullup is off, set SOFT_DISCONNECT

This is especially useful when a gadget disconnection is initiated but
no board_usb_cleanup() is called.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>