usb: gadget: mv_udc: refine suspend/resume function

This patch impletments system suspend/resume functions for Marvell
otg controller.
If OTG is enabled, OTG driver will do most of the work.
If not, we will check clock gating.
  If clock gating is enabled, the UDC will be start/stop automatically.
  If not, UDC will be start/stop in suspend/resume functions.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>