usb: device_next: Add Kconfig to set maximum speed

Two main ideas behind setting maximum speed are:
  * Allow code and RAM optimizations at compile time
  * Allow High-Speed capable drivers to limit operating speed to user
    choice.

This commit only introduces the necessary Kconfig options but does not
implement any code or RAM optimizations and does not modify any driver.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>