dt-bindings: property-units: Add picoseconds type

Add the '-ps' picosecond unit suffix for property names.
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/property-units.txt

@@ -17,6 +17,7 @@ Time/Frequency
 -ms		: millisecond
 -us		: microsecond
 -ns		: nanosecond
+-ps		: picosecond
 
 Distance
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