periph_drivers: Proposal for a minimal RTC interface

This is a proposal for a low-level RTC driver. (now for real)
It tries to follow the principles of @haukepetersen's low-level driver interfaces and is open for discussion.
```
 * The values in `tm` are expected to conform to the `struct tm` specification.
 * Compare: http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html
```
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