thunderbolt: Add 'generation' attribute for devices

The Thunderbolt standard went through several major iterations, here
called generation. USB4, which will be based on Thunderbolt, will be
generation 4. Let userspace know the generation of the controller in
the devices in order to distinguish between Thunderbolt and USB4, so
it can be shown in various user interfaces.

Signed-off-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>