include/drivers: Add RTC API header and handlers

This commit adds the rtc.h header file which contains
API functions for real-time-clocks, which are low power
devices which track and represent broken-down time.

It also changes one line of doxygen documentation in the
maxim_ds3132.h file to place it in its own group.

The handlers for use of the API from userspace is also
added with this commit.

The API is split into one mandatory section, setting and
getting time, and three optional sections, alarms, update
event callback, and clock calibration.

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