drivers: gnss: Add emulated GNSS device driver

The emulated GNSS driver behaves like a GNSS, implementing
device pm and the GNSS APIs, using only kernel features (
zephyr work queue and uptime) making it buildable on all
zephyr targets.

The purpose of this device driver is to tailor and validate
the gnss api test suite.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>