usb: dwc2: gadget: powerup controller if needed

During vbus session, usb controller needs to exit hibernation if it was
previously in suspend state.
Since controller will be resetted and configured, there is no need
to restore registers.

Moreover, set lx_state to L0 on B session. vbus_session callback may
not be used by all platforms. Thus, controller software state needs
to be set to L0 if the controller detects a valid B session.
Otherwise, lx_state will remain L2 and prevent any request submission.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>