drivers: modem: gsm: wait for cellular registration

The driver should check for cellular registration before
attempting to attach to packet service, otherwise it will just
fails.

This patch waits for registration for 300 seconds, configurable
by CONFIG_MODEM_GSM_REGISTER_TIMEOUT, if it isn't registered by
then, it would toggle the airplane mode using AT+CFUN & wait
again, until it is registered, or `gsm_ppp_stop` is invoked.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>