rtc: Make the API ready for multiple RTC instances

A platform might provide multiple RTC, either internally or externally.
Applying the changes to the DesignWare driver relevantly as well as
to the sample code.

Change-Id: Ia70e791a6c45e186cbc4dc900a268fa882331af5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
5 files changed