doc: describe the scp03 command

The Secure Channel Protocol 03 command sends control requests
(enable/provision) to the TEE implementing the protocol between the
processor and the secure element.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/index.rst

@@ -33,3 +33,4 @@ Shell commands
    qfw
    sbi
    true
+   scp03

doc/usage/scp03.rst

@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+scp03 command
+=============
+
+Synopsis
+--------
+
+::
+
+    scp03 enable
+    scp03 provision
+
+Description
+-----------
+
+The *scp03* command calls into a Trusted Application executing in a
+Trusted Execution Environment to enable (if present) the Secure
+Channel Protocol 03 stablished between the processor and the secure
+element.
+
+This protocol encrypts all the communication between the processor and
+the secure element using a set of pre-defined keys. These keys can be
+rotated (provisioned) using the *provision* request.
+
+See also
+--------
+
+For some information on the internals implemented in the TEE, please
+check the GlobalPlatform documentation on `Secure Channel Protocol '03'`_
+
+.. _Secure Channel Protocol '03':
+   https://globalplatform.org/wp-content/uploads/2014/07/GPC_2.3_D_SCP03_v1.1.2_PublicRelease.pdf