NFC: st21nfca: Auto-select core module

The core st21nca module is useless without the I2C access module.
So hide NFC_ST21NFCA and select it automatically if either
NFC_ST21NFCA_I2C is selected.

This avoids presenting NFC_ST21NFCA when NFC_ST21NFCA_I2C can't be
selected.

Cc: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>