thunderbolt: Add support for XDomain properties

Thunderbolt XDomain discovery protocol uses directories which contain
properties and other directories to exchange information about what
capabilities the remote host supports. This also includes identification
information like device ID and name.

This adds support for parsing and formatting these properties and
establishes an API drivers can use in addition to the core Thunderbolt
driver. This API is exposed in a new header: include/linux/thunderbolt.h.

This code is based on the work done by Amir Levy and Michael Jamet.

Signed-off-by: Michael Jamet <michael.jamet@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>