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/change-log.rst b/docs/change-log.rst
index 11fcf21..73e6d72 100644
--- a/docs/change-log.rst
+++ b/docs/change-log.rst
@@ -969,7 +969,7 @@
    fixed sample points at key places in the code.
 
 -  To support the QEMU platform port, imported libfdt v1.4.1 from
-   https://git.kernel.org/cgit/utils/dtc/dtc.git
+   https://git.kernel.org/pub/scm/utils/dtc/dtc.git
 
 -  Updated PSCI support: