doc: Treat Sphinx warnings as errors

'make doc' will now fail if Sphinx outputs any warning messages during
documentation generation.

Change-Id: I3e466af58ccf29b14a7e61037539b79ab6fc6037
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/Makefile

 #
-# Copyright (c) 2019, ARM Limited. All rights reserved.
+# Copyright (c) 2019-2020, ARM Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -7,7 +7,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -W
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = TrustedFirmware-A
 SOURCEDIR     = .