tanaro:rtcs: Fix to RTC overlay mechanism

The first approach of selecting a RTC didn't work as expected.
The active RTC must reside on the rtc0 alias because otherwise
the hwclock utilities can't access the RTC.
For this to work, we now have a placeholder in the main device tree
that is filled with the actual values in one of the two overlays.
This breaks with the naming conventions for I2C bus devices, but
right now, we don't have a better solution.

BCS 746-000877

(cherry picked from commit 11963a85)