boards/posix/native_posix: Add emulated RTC device driver

The emulated RTC device driver is used to emulate a real
RTC device. Note that it is not a replacement for the
native_rtc module, which is used to control simulated time,
get time from the host system, etc.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>