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/allwinner.rst b/docs/plat/allwinner.rst
index a1e0659..d82380d 100644
--- a/docs/plat/allwinner.rst
+++ b/docs/plat/allwinner.rst
@@ -34,7 +34,7 @@
 
     make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_h6 DEBUG=1 bl31
 
-.. _U-Boot documentation: http://git.denx.de/?p=u-boot.git;f=board/sunxi/README.sunxi64;hb=HEAD
+.. _U-Boot documentation: https://gitlab.denx.de/u-boot/u-boot/-/blob/master/board/sunxi/README.sunxi64
 
 Trusted OS dispatcher
 ---------------------