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>