refclock: add new refclock for RTCs

refclock: add new refclock for RTCs

This refclock uses an RTC as reference source. If the RTC doesn't
support reporting an update event this source is quite coarse as it
usually needs a slow bus access to be read and has a precision of only
one second. If reporting an update event is available, the time is read
just after such an event which improves precision.

Depending on hardware capabilities you might want to combine it with a
PPS reference clock sourced from the same chip.

Note that you can enable UIE emulation in the Linux kernel to make a RTC
without interrupt support look like one with irqs in return for some
system and bus overhead.

Co-authored-by: Ahmad Fatoum <a.fatoum@pengutronix.de>