usb: dwc2: Add clock gating entering flow by system suspend

If not hibernation nor partial power down are supported,
clock gating is used to save power.

Adds a new flow of entering clock gating when PC is
suspended.

Signed-off-by: Artur Petrosyan <Arthur.Petrosyan@synopsys.com>
Link: https://lore.kernel.org/r/20210413073716.30C13A0094@mailhost.synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>