drivers: Add GNSS API public header

This commit adds the public header for the GNSS API, along
with the initial GNSS Kconfig file and an entry in the
common linker file for registered GNSS data callbacks.

A very naive implementation of the GNSS data callback is
provided as well in drivers/gnss/gnss_publish.c

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