usb: dwc2: Add support for 'maximum-speed' property

usb: dwc2: Add support for 'maximum-speed' property

The DWC2 IP can be properly integrated in a SoC to work at high-speed
USB speed but some board issues, EMC constraints or any other reasons
can lead to the need to limit this USB speed at board level.

The device-tree 'maximum-speed' property already exists for this purpose
but is not handled by the DWC2 driver.

Fill this lack adding support for 'maximum-speed' property and so allow
to limit the USB speed in device-tree (board description).

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20250910160730.585303-1-herve.codina@bootlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>