Let the core handle offsetting and windowing the RTC range.

The RTC has a 40-bit counter counting at 1024 Hz. So its maximum value is
2^(40-10) - 1. Also, let the core handle the offset instead of coding it in
the callbacks. Keep the default epoch at the beginning of 2009 (this will
fail in 2043).
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>