Some modems supports multiple access technologies.
The access technology of the serving cell can be extracted from the
+CEREG command, which is already sent to the modem.
The parse function is extended to also parse the access technology value.
Fixes https://github.com/zephyrproject-rtos/zephyr/issues/51153.
Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>