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>