thunderbolt: debugfs: Add USB4 Gen 4 margining capabilities

Parse the Gen 4 specific capabilities. Change the return types of
independent_voltage_margins() and independent_time_margins() to enums
that distinguish between the Gen 2/3 and Gen 4 margins since they behave
differently between generations.

Signed-off-by: Aapo Vienamo <aapo.vienamo@iki.fi>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>