thunderbolt: Add sideband register access to debugfs

This makes it possible to read and write USB4 port and retimer sideband
registers through debugfs which is useful for debugging and manufacturing
purposes. We add "sb_regs" debugfs attribute under each USB4 port and
retimer that is used to access the sideband.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>