usb: udc: Introduce ->udc_set_speed() method

This patch was copied from kernel commit: 67fdfda4a99ed.

Sometimes, the gadget driver we want to run has max_speed lower than
what the UDC supports. In such situations, UDC might want to make sure
we don't try to connect on speeds not supported by the gadget
driver because that will just fail.

So here introduce a new optional ->udc_set_speed() method which can be
implemented by interested UDC drivers to achieve this purpose.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>