usb: dwc2: Add STM32 related debugfs entries

These are entries related to STM32MP1 PHY control.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Martin Devera <devik@eaxlabs.cz>
Link: https://lore.kernel.org/r/20210227215528.12752-1-devik@eaxlabs.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>