NFC: trf7970a: Add VIN voltage override support

The trf7970a driver uses the voltage from the
power/regulator subsystem to determine what the
voltage on the VIN pin is.  Normally, this is
the right thing to do but sometimes the board
that the trf7970a is on may change the voltage.
This is the case for the trf7970atb board from
Texas Instruments where it boosts the VIN voltage
from 3.3V to 5V (see http://www.ti.com/tool/trf7970atb

).

To handle this, add support for the 'vin-voltage-override'
device tree property which overrides the voltage value
given by the regulator subsystem.  When the DT property
is not present, the value from the regulator subsystem
is used.

The value of 'vin-voltage-override' is in uVolts.

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