FROMGIT: 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>
(cherry picked from commit f228cb27
 https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git

 usb-next)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2afe995aa0eb3b2c3f1e271f86bb44784a26dda8