usb: phy: phy-generic: add the implementation of .set_suspend

Add clock operation at .set_suspend if the PHY has
suspend requirement, it can be benefit of power saving for
phy and the whole system (parent clock may also be disabled).

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>