usb: dwc2: Enable power down

Enable the power down option based on the core capability.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: Artur Petrosyan <arturp@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>