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>

doc/chrony.conf.adoc

@@ -478,7 +478,7 @@ the driver-specific parameter using the *:* character.
 +
 This directive can be used multiple times to specify multiple reference clocks.
 +
-There are four drivers included in *chronyd*:
+There are five drivers included in *chronyd*:
 +
 *PPS*:::
 Driver for the kernel PPS (pulse per second) API. The parameter is the path to
@@ -596,6 +596,23 @@ refclock PHC /dev/ptp1:nocrossts poll 3 pps
 refclock PHC /dev/ptp2:extpps:pin=1 width 0.2 poll 2
 ----
 +
+*RTC*:::
+Driver for using the Real Time Clock (RTC) as a reference clock. The parameter
+is the path to the RTC character device which should be used as a time source.
+This driver cannot be used together with the <<rtcfile,*rtcfile*>> or
+<<rtcsync,*rtcsync*>> directive. The driver supports the following option:
++
+*utc*::::
+Assume that RTC keeps Universal Coordinated Time (UTC) instead of local
+time.
+{blank}:::
++
+Examples:
++
+----
+refclock RTC /dev/rtc0:utc
+----
++
 {blank}::
 The *refclock* directive supports the following options:
 +
@@ -2206,7 +2223,8 @@ The directive takes no arguments. It is equivalent to specifying the *-u*
 switch to the Linux *hwclock* program.
 +
 Note that this setting is overridden by the <<hwclockfile,*hwclockfile*>> file
-and is not relevant for the <<rtcsync,*rtcsync*>> directive.
+and is not relevant for the <<rtcsync,*rtcsync*>> directive or when the RTC
+is used as reference clock.
 
 [[rtcsync]]*rtcsync*::
 The *rtcsync* directive enables a mode where the system time is periodically