dwc3: Add support for clock disabling during suspend

dwc3: Add support for clock disabling during suspend

In dwc3 usb controller, when remote wakeup feature is enabled and
the controller is in U3 state (suspended), it requires suspend_clk
to be active to detect wakeup signalling driven on the link.
For all other cases usb clocks can be disabled once the controller
enters U3 state.

This patch adds the logic for disabling usb clocks when remote
wakeup feature is not enabled.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>