devicetree: property-units: Add femtofarads unit

When dealing with capacitance of 0.5 pF then
a smaller unit is preferred.
Add femtofarads to deal with this.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/property-units.txt

@@ -31,6 +31,7 @@ Electricity
 -microwatt-hours: micro Watt-hours
 -microvolt	: micro volts
 -picofarads	: picofarads
+-femtofarads	: femtofarads
 
 Temperature
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