doc: man-page for gpio command

Provide a man-page for the gpio command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/gpio.rst

@@ -0,0 +1,90 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+gpio command
+============
+
+Synopsis
+--------
+
+::
+
+    gpio <input|set|clear|toggle> <pin>
+    gpio read <name> <pin>
+    gpio status [-a] [<bank>|<pin>]
+
+The gpio command is used to access General Purpose Inputs/Outputs.
+
+gpio input
+----------
+
+Switch the GPIO *pin* to input mode.
+
+gpio set
+--------
+
+Switch the GPIO *pin* to output mode and set the signal to 1.
+
+gpio clear
+----------
+
+Switch the GPIO *pin* to output mode and set the signal to 0.
+
+gpio toggle
+-----------
+
+Switch the GPIO *pin* to output mode and reverse the signal state.
+
+gpio read
+---------
+
+Read the signal state of the GPIO *pin* and save it in environment variable
+*name*.
+
+gpio status
+-----------
+
+Display the status of one or multiple GPIOs. By default only claimed GPIOs
+are displayed.
+
+-a
+    Display GPIOs irrespective of being claimed.
+
+bank
+    Name of a bank of GPIOs to be displayed.
+
+pin
+    Name of a single GPIO to be displayed or manipulated.
+
+Examples
+--------
+
+Switch the status of a GPIO::
+
+    => gpio set a5
+    gpio: pin a5 (gpio 133) value is 1
+    => gpio clear a5
+    gpio: pin a5 (gpio 133) value is 0
+    => gpio toggle a5
+    gpio: pin a5 (gpio 133) value is 1
+    => gpio read myvar a5
+    gpio: pin a5 (gpio 133) value is 1
+    => echo $myvar
+    1
+    => gpio toggle a5
+    gpio: pin a5 (gpio 133) value is 0
+    => gpio read myvar a5
+    gpio: pin a5 (gpio 133) value is 0
+    => echo $myvar
+    0
+
+Configuration
+-------------
+
+The *gpio* command is only available if CONFIG_CMD_GPIO=y.
+The *gpio read* command is only available if CONFIG_CMD_GPIO_READ=y.
+
+Return value
+------------
+
+If the command succeds the return value $? is set to 0. If an error occurs, the
+return value $? is set to 1.

doc/usage/index.rst

@@ -45,6 +45,7 @@ Shell commands
    cmd/fatload
    cmd/fdt
    cmd/for
+   cmd/gpio
    cmd/load
    cmd/loadm
    cmd/loady