drivers: gnss: Add GNSS driver for Quectel LCX6G

This commit adds a GNSS driver for the Quectel LCX6G
series of GNSS modems (LC26G, LC76G, LC86G). It is
based on the modem subsystem, and the GNSS utilities
added in the two previous commits.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>