usb: dwc2: Add host clock gating support functions

Added host clock gating support functions according
programming guide.

Added function names:
dwc2_host_enter_clock_gating()
dwc2_host_exit_clock_gating()

Acked-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Signed-off-by: Artur Petrosyan <Arthur.Petrosyan@synopsys.com>
Link: https://lore.kernel.org/r/20210413073615.B3E84A022E@mailhost.synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>