pwm: Drop documentation for time_unit

This was dropped in commit 4246bc9 ("pwm: Drop support for time
units other than nanoseconds").

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>

Author: ukleinek

Documentation/driver-api/pwm.rst

@@ -115,34 +115,23 @@ channel that was exported. The following properties will then be available:
 
   period
     The total period of the PWM signal (read/write).
-    Value the sum of the active and inactive time of the PWM, using the
-    time_unit property as a measurment unit.
+    Value is in nanoseconds and is the sum of the active and inactive
+    time of the PWM.
 
   duty_cycle
     The active time of the PWM signal (read/write).
-    Value must be less than the period and uses the time_unit property as
-    a measurment unit.
+    Value is in nanoseconds and must be less than the period.
 
   phase
     The phase difference between the actual PWM signal and the reference one.
-    The value must be lower than period and uses the time_unit property as a
-    measurment unit.
+    The value is expressed in nanoseconds and must be lower than period.
 
   polarity
     Changes the polarity of the PWM signal (read/write).
     Writes to this property only work if the PWM chip supports changing
     the polarity. The polarity can only be changed if the PWM is not
     enabled. Value is the string "normal" or "inversed".
 
-  time_unit
-    The time unit used to measure the pwm period, duty_cycle and phase.
-
-  - 1 - seconds
-  - 2 - miliseconds
-  - 3 - microseconds
-  - 4 - nanoseconds
-  - 5 - picoseconds
-
   enable
     Enable/disable the PWM signal (read/write).