usb: dwc3: Use clock API to control the memory power states

GDSC driver is removing control for USB memory power states.
DWC driver need to explicitly call clk_set_flag() API to turn
off both memory core and periphery upon vbus off. When core
clock is turned on both of them will be turn on automatically.

Change-Id: I7e442daf51c6ece39b8fe5cc8a38d3405163c9b5
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>