usb: dwc2: enable FS/LS PHY clock select on STM32MP15 FS OTG

When the core is in FS host mode, using the FS transceiver, and a Low-Speed
device is connected, transceiver clock is 6Mhz.
So, to support Low-Speed devices, enable support of FS/LS Low Power mode,
so that the PHY supplies a 6 MHz clock during Low-Speed mode.

Acked-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20210105094855.30763-3-amelie.delaunay@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>