Add sp environment documentation

Explain differences between opteesp and sp environments, along
documenting the build process of sp deployments.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I036c30e730c355c134d3167f44723cd3a400bf77
diff --git a/docs/developer/software-requirements.rst b/docs/developer/software-requirements.rst
index 1449724..cbdf81a 100644
--- a/docs/developer/software-requirements.rst
+++ b/docs/developer/software-requirements.rst
@@ -16,7 +16,7 @@
    * Python3.6 and the modules listed in ``<project>/requirements.txt``.
    * GCC supporting the deployment.
 
-       * `opteesp` environment: a host to aarch64 cross-compiler is needed. Please use the compilers specified by the
+       * `opteesp` and `sp` environments: a host to aarch64 cross-compiler is needed. Please use the compilers specified by the
          `OP-TEE documentation`_.
        * `arm-linux` environment: a host to aarch64 linux cross-compiler is needed. Please use the version `9.2-2019.12` of the
          "aarch64-none-linux-gnu" compiler available from `arm Developer`_.
@@ -34,6 +34,6 @@
 .. _CMake download page: https://cmake.org/files/v3.18/
 .. _`AEM FVP`: https://developer.arm.com/-/media/Files/downloads/ecosystem-models/FVP_Base_RevC-2xAEMvA_11.18_16_Linux64.tgz
 
-*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2023, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause