NFC: trf7970a: Add Target Mode Detection Support

Add the ability to detect the mode (i.e., RF technology)
used by the initiator.  The RF technology that was
detected can be retrieved by calling the 'tg_get_rf_tech'
driver hook.

Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>