docs: dt: remove stale property-units.txt, link to current schema

property-units.txt is not maintained anymore. New suffixes are now
supported that have not been added here.

To avoid people incorrectly think a given suffix is not in the list remove
the file entirely and point to the authoritative place where the
recommended suffixes are maintained.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210512152311.7399-1-luca@lucaceresoli.netSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/property-units.txt

-Standard Unit Suffixes for Property names
-
-Properties which have a unit of measure are recommended to have a unit
-suffix appended to the property name. The list below contains the
-recommended suffixes. Other variations exist in bindings, but should not
-be used in new bindings or added here. The inconsistency in the unit
-prefixes is due to selecting the most commonly used variants.
-
-It is also recommended to use the units listed here and not add additional
-unit prefixes.
-
-Time/Frequency
-----------------------------------------
--mhz		: megahertz
--hz		: hertz (preferred)
--sec		: second
--ms		: millisecond
--us		: microsecond
--ns		: nanosecond
--ps		: picosecond
-
-Distance
-----------------------------------------
--mm		: millimeter
-
-Electricity
-----------------------------------------
--microamp	: microampere
--microamp-hours : microampere hour
--ohms		: ohm
--micro-ohms	: microohm
--microwatt-hours: microwatt hour
--microvolt	: microvolt
--picofarads	: picofarad
--femtofarads	: femtofarad
-
-Temperature
-----------------------------------------
--celsius	: degree Celsius
--millicelsius	: millidegree Celsius
-
-Pressure
-----------------------------------------
--kpascal	: kilopascal
-
-Throughput
-----------------------------------------
--kBps		: kilobytes per second

Documentation/devicetree/bindings/writing-bindings.rst

@@ -52,7 +52,8 @@ Properties
   constraints specific to the device.
 
 - DO use common property unit suffixes for properties with scientific units.
-  See property-units.txt.
+  Recommended suffixes are listed at
+  https://github.com/devicetree-org/dt-schema/blob/master/schemas/property-units.yaml
 
 - DO define properties in terms of constraints. How many entries? What are
   possible values? What is the order?