Update links to TF-A documentation

Update the links to the TF-A documentation to point to
trustedfirmware.org rather than Github.

Change-Id: Ib49b90b6733cdf6acd17d95e6d4968ac162bd150
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index 0decf1a..6a5abb7 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -449,5 +449,5 @@
 
 *Copyright (c) 2018-2019, Arm Limited. All rights reserved.*
 
-.. _docs/psci-pd-tree.rst: https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/psci-pd-tree.rst
+.. _docs/psci-pd-tree.rst: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/about/docs/psci-pd-tree.rst
 .. _SP805: https://static.docs.arm.com/ddi0270/b/DDI0270.pdf