NFC: st21nfca: Covert to use GPIO descriptor

Since we got rid of platform data, the driver may use GPIO descriptor
directly.

Looking deeply to the use of the GPIO pin it looks like it should be
a fixed voltage regulator rather than custom GPIO handling. But this
is out of scope of the change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>