modem_cellular: add RSRP & RSRQ to U-blox SARA-R5 modem

This commit implements parsing of the CESQ extended signal quality AT
command, extracting RSRP and RSRQ which is relevant for LTE connections.
It's used in the U-blox SARA-R5 modem instance. Furthermore, the IMEI,
IMSI is extracted in the same modem.

Signed-off-by: Emil Lindqvist <emil@lindq.gr>