dt/bindings: add documentation on standard property unit suffixes

A common review comment is to add units to DT property names. The naming
convention is inconsistent unfortunately although there are clear
majorities in use already. This makes it hard to remember the exact name
preferred, so I'm documenting this to provide an easy point of
reference.
Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.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		: seconds
+-ms		: milliseconds
+-us		: microseconds
+-ns		: nanoseconds
+
+Distance
+----------------------------------------
+-mm		: millimeters
+
+Electricity
+----------------------------------------
+-microamp	: micro amps
+-ohms		: Ohms
+-micro-ohms	: micro Ohms
+-microvolt	: micro volts
+
+Temperature
+----------------------------------------
+-celsius	: Degrees Celsius
+-millicelsius	: Degreee milli-Celsius
+
+Pressure
+----------------------------------------
+-kpascal	: kiloPascal