doc: usage: add usage details for reset cmd

Add usage details for reset command.

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>

doc/usage/index.rst

@@ -40,3 +40,4 @@ Shell commands
    sbi
    true
    scp03
+   reset

doc/usage/reset.rst

@@ -0,0 +1,26 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+reset command
+=============
+
+Synopsis
+--------
+
+::
+
+    reset [-w]
+
+Description
+-----------
+
+Perform reset of the CPU. By default does COLD reset, which resets CPU,
+DDR and peripherals, on some boards also resets external PMIC.
+
+-w
+    Do warm WARM, reset CPU but keep peripheral/DDR/PMIC active.
+
+
+Return value
+------------
+
+The return value $? is always set to 0 (true).