drivers: modem: introducing config for cereg/creg for gsm

This contribution addresses the support for various types of modems in
gsm driver. As some 4G modems have failed to return correct output
from AT+CREG?, so AT+CEREG? is the right AT command in such situation.
This commit provides the possibility for user to select one type of
AT command. This PR fixes zephyrproject-rtos#63917

Signed-off-by: Tahir Akram <mtahirbutt@hotmail.com>