cyclic: Add documentation

Add documentation for the cyclic function infrastructure, including the
cyclic command.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/cyclic.rst

@@ -0,0 +1,45 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+cyclic command
+==============
+
+Synopsis
+--------
+
+::
+
+    cyclic list
+
+Description
+-----------
+
+The cyclic list command provides a list of the currently registered
+cyclic functions.
+
+This shows the following information:
+
+Function
+    Function name
+
+cpu-time
+    Total time spent in this cyclic function.
+
+Frequency
+    Frequency of execution of this function, e.g. 100 times/s for a
+    pediod of 10ms.
+
+
+See :doc:`../../develop/cyclic` for more information on cyclic functions.
+
+Example
+-------
+
+::
+
+    => cyclic list
+    function: cyclic_demo, cpu-time: 52906 us, frequency: 99.20 times/s
+
+Configuration
+-------------
+
+The cyclic command is only available if CONFIG_CMD_CYCLIC=y.