doc: man-page for the wdt command

Describe the wdt command.

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

doc/usage/index.rst

@@ -52,3 +52,4 @@ Shell commands
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+   wdt

doc/usage/wdt.rst

+.. SPDX-License-Identifier: GPL-2.0+:
+
+wdt command
+============
+
+Synopsis
+--------
+
+::
+
+    wdt list
+    wdt dev [<name>]
+    wdt start <timeout_ms> [flags]
+    wdt stop
+    wdt reset
+    wdt expirer [flags]
+
+Description
+-----------
+
+The wdt command is used to control watchdog timers.
+
+The 'wdt list' command shows a list of all watchdog devices.
+
+The 'wdt dev' command called without argument shows the current watchdog device.
+The current device is set when passing the name of the device as argument.
+
+The 'wdt start' command starts the current watchdog timer.
+
+The 'wdt stop' command stops the current watchdog timer.
+
+The 'wdt reset' command resets the current watchdog timer without stopping it.
+
+The 'wdt expire' command let's the current watchdog timer expire immediately.
+This will lead to a reset.
+
+name
+    name of the watchdog device
+
+timeout_ms
+    timeout interval in milliseconds
+
+flags
+    unsigned long value passed to the driver. The usage is driver specific.
+    The value is ignored by most drivers.
+
+Example
+-------
+
+::
+
+    => wdt dev
+    No watchdog timer device set!
+    => wdt list
+    watchdog@1c20ca0 (sunxi_wdt)
+    => wdt dev watchdog@1c20ca0
+    => wdt dev
+    dev: watchdog@1c20ca0
+    => wdt start 3000
+    => wdt reset
+    => wdt stop
+    => wdt expire
+
+    U-Boot SPL 2022.04-rc3 (Mar 25 2022 - 13:48:33 +0000)
+
+ In the example above '(sunxi_wdt)' refers to the driver for the watchdog
+ device.
+
+Configuration
+-------------
+
+The command is only available if CONFIG_CMD_WDT=y.
+
+Return value
+------------
+
+The return value $? is 0 if the command succeeds, 1 upon failure.