doc: document the pwm command

Signed-off-by: Emil Kronborg <emil.kronborg@protonmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/pwm.rst

+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+.. index::
+   single: pwm (command)
+
+pwm command
+===========
+
+Synopsis
+--------
+
+::
+
+    pwm invert <pwm_dev_num> <channel> <polarity>
+    pwm config <pwm_dev_num> <channel> <period_ns> <duty_ns>
+    pwm enable <pwm_dev_num> <channel>
+    pwm disable <pwm_dev_num> <channel>
+
+
+Description
+-----------
+
+The ``pwm`` command is used to access and configure PWM (Pulse Width Modulation)
+signals.
+
+pwm invert
+----------
+
+* If the value of ``polarity`` is 0, the default polarity is used.
+* If the value of ``polarity`` is 1, the polarity is inverted.
+
+pwm config
+----------
+
+Configure the period and duty period in nanoseconds.
+
+pwm enable
+----------
+
+Enable output on the configured device and channel.
+
+pwm disable
+-----------
+
+Disable output on the configured device and channel.
+
+pwm_dev_num
+    Device number of the pulse width modulation device
+
+channel
+    Output channel of the PWM device
+
+polarity
+    * 0 - Use normal polarity
+    * 1 - Use inverted polarity
+
+duty_ns
+    Duty period in ns
+
+period_ns
+    Period time in ns
+
+Examples
+--------
+
+Configure device 0, channel 0 to 20 µs period and 14 µs (that is, 70%) duty period::
+
+    => pwm config 0 0 20000 14000
+
+Enable output on the configured device and channel::
+
+    => pwm enable 0 0
+
+Disable output on the configured device and channel::
+
+    => pwm disable 0 0
+
+Invert the signal on the configured device and channel::
+
+    => pwm invert 0 0 1
+
+Configuration
+-------------
+
+The ``pwm`` command is only available if CONFIG_CMD_PWM=y.
+
+Return value
+------------
+
+If the command succeeds, the return value ``$?`` is set to 0. If an error occurs, the
+return value ``$?`` is set to 1.

doc/usage/index.rst

@@ -93,6 +93,7 @@ Shell commands
    cmd/pinmux
    cmd/printenv
    cmd/pstore
+   cmd/pwm
    cmd/qfw
    cmd/read
    cmd/reset