doc: Fix some broken links

Fix all external broken links reported by Sphinx linkcheck tool.

This does not take care of broken cross-references between internal
TF-A documentation files. These will be fixed in a future patch.

Change-Id: I2a740a3ec0b688c14aad575a6c2ac71e72ce051e
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/plat/meson-axg.rst b/docs/plat/meson-axg.rst
index 1e4b2c2..6f6732e 100644
--- a/docs/plat/meson-axg.rst
+++ b/docs/plat/meson-axg.rst
@@ -24,4 +24,4 @@
 instructions in the `U-Boot repository`_, replacing the mentioned **bl31.img**
 by the one built from this port.
 
-.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/board/amlogic/s400/README
+.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/doc/board/amlogic/s400.rst