thunderbolt: Expose router DROM through debugfs

Router DROM contains information that might be usable for development
and debugging purposes. For example when new entries are added to the
USB4 spec it is useful to be able to look for them without need to
change the kernel.

For this reason expose the DROM through debugfs.

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