doc: Fix broken external links

Using Sphinx linkcheck on the TF-A docs revealed some broken
or permanently-redirected links. These have been updated where
possible.

Change-Id: Ie1fead47972ede3331973759b50ee466264bd2ee
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/platform-compatibility-policy.rst b/docs/platform-compatibility-policy.rst
index b33c4e7..d7e13ad 100644
--- a/docs/platform-compatibility-policy.rst
+++ b/docs/platform-compatibility-policy.rst
@@ -41,5 +41,5 @@
 *Copyright (c) 2018, Arm Limited and Contributors. All rights reserved.*
 
 .. _Porting Guide: ./porting-guide.rst
-.. _Release information: https://github.com/ARM-software/arm-trusted-firmware/wiki/TF-A-Release-information#2removal-of-deprecated-interfaces
+.. _Release information: https://github.com/ARM-software/arm-trusted-firmware/wiki/TF-A-Release-information#removal-of-deprecated-interfaces
 .. _tf-issue: https://github.com/ARM-software/tf-issues/issues