NFC: trf7970a: Remove support for 'vin-voltage-override' DT property

The 'vin-voltage-override' DT property is used by the trf7970a
driver to override the voltage presented to the driver by the
regulator subsystem.  This is unnecessary as properly specifying
the regulator chain via DT properties will accomplish the same
thing.  Therefore, remove support for 'vin-voltage-override'.

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