This is needed by the new ICM interface to find xdomains by route string
instead of link and depth.

While there update existing tb_xdomain_find_* functions to use
tb_xdomain_get() instead of open-coding the same.
Signed-off-by: NRadion Mirchevsky <radion.mirchevsky@intel.com>
Signed-off-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>