usb: roles: Add a description for the class to Kconfig

That makes the USB role switch support option visible and
selectable for the user. The class driver is also moved to
drivers/usb/roles/ directory.

This will fix an issue that we have with the Intel USB role
switch driver on systems that don't have USB Type-C connectors:

Intel USB role switch driver depends on the USB role switch
class as it should, but since there was no way for the user
to enable the USB role switch class, there was also no way
to select that driver. USB Type-C drivers select the USB
role switch class which makes the Intel USB role switch
driver available and therefore hides the problem.

So in practice Intel USB role switch driver was depending on
USB Type-C drivers.

Fixes: f6fb9ec0 ("usb: roles: Add Intel xHCI USB role switch driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>