usb: device_next: rework USBD shell to take speed into account

Rework USBD shell to take speed arguments into account and revise
command help text. Rename some of the commands to better reflect what
they actually do.
Since most of the init code changes, rewrite the command to initialize
the default configuration in the shell to register all available
classes, but not to enable USB device support in a single command.
This will allow earlier notification in the future, if supported by the
controller, before the USB device controller is enabled.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>