[CPUFREQ] Document units for transition latency

They're documented in the header but not in Documentation.

Signed-off-by: Mark Brown <[e-mail]>
Signed-off-by: Dave Jones <[e-mail]>

Documentation/cpu-freq/cpu-drivers.txt

···
 				(in kHz) which is supported by 
 				this CPU
 policy->cpuinfo.transition_latency   the time it takes on this CPU to
-				switch between two frequencies (if
-				appropriate, else specify
-				CPUFREQ_ETERNAL)
+				switch between two frequencies in
+				nanoseconds (if appropriate, else
+				specify CPUFREQ_ETERNAL)
 
 policy->cur			The current operating frequency of
 				this CPU (if appropriate)