dt-bindings: rtc: Add quartz-load-femtofarads property

The quartz-load-femtofarads are relevant for
several users. Add it as a common property in rtc.
Note that valid values and default values must be documented.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Suggested-by: Rob Herring <robh+dt@kernel.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/rtc.txt

@@ -21,12 +21,16 @@ Optional properties
 The following properties may not be supported by all drivers. However, if a
 driver wants to support one of the below features, it should adapt the bindings
 below.
-- trickle-resistor-ohms : Selected resistor for trickle charger. Should be given
-                          if trickle charger should be enabled
-- trickle-diode-disable : Do not use internal trickle charger diode Should be
-                          given if internal trickle charger diode should be
-                          disabled
-- wakeup-source :         Enables wake up of host system on alarm
+- trickle-resistor-ohms :   Selected resistor for trickle charger. Should be given
+                            if trickle charger should be enabled
+- trickle-diode-disable :   Do not use internal trickle charger diode Should be
+                            given if internal trickle charger diode should be
+                            disabled
+- wakeup-source :           Enables wake up of host system on alarm
+- quartz-load-femtofarads : The capacitive load of the quartz(x-tal),
+                            expressed in femto Farad (fF).
+                            The default value shall be listed (if optional),
+                            and likewise all valid values.
 
 Trivial RTCs
 ------------