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>

docs/perf/psci-performance-juno.rst

@@ -286,7 +286,7 @@ effects, given that these measurements are at the nano-second level.
 
 --------------
 
-*Copyright (c) 2019, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2019-2020, Arm Limited and Contributors. All rights reserved.*
 
-.. _Juno R1 platform: https://www.arm.com/files/pdf/Juno_r1_ARM_Dev_datasheet.pdf
+.. _Juno R1 platform: https://static.docs.arm.com/100122/0100/arm_versatile_express_juno_r1_development_platform_(v2m_juno_r1)_technical_reference_manual_100122_0100_05_en.pdf
 .. _TF master as of 31/01/2017: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/tree/?id=c38b36d

docs/plat/allwinner.rst

@@ -34,7 +34,7 @@ To build for machines with an H6 SoC:
 
     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
 ---------------------

docs/plat/meson-axg.rst

@@ -24,4 +24,4 @@ This port has been tested on a A113D board. After building it, follow the
 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

docs/plat/meson-g12a.rst

@@ -20,8 +20,8 @@ In order to build it:
     CROSS_COMPILE=aarch64-linux-gnu- make DEBUG=1 PLAT=g12a
 
 This port has been tested on a SEI510 board. After building it, follow the
-instructions in the `gxlimg repository` or `U-Boot repository`_, replacing the
+instructions in the `gxlimg repository`_ or `U-Boot repository`_, replacing the
 mentioned **bl31.img** by the one built from this port.
 
 .. _gxlimg repository: https://github.com/repk/gxlimg/blob/master/README.g12a
-.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/board/amlogic/sei510/README
+.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/doc/board/amlogic/sei510.rst

docs/plat/meson-gxl.rst

@@ -20,8 +20,8 @@ In order to build it:
     CROSS_COMPILE=aarch64-linux-gnu- make DEBUG=1 PLAT=gxl
 
 This port has been tested on a Lepotato. After building it, follow the
-instructions in the `gxlimg repository` or `U-Boot repository`_, replacing the
+instructions in the `gxlimg repository`_ or `U-Boot repository`_, replacing the
 mentioned **bl31.img** by the one built from this port.
 
 .. _gxlimg repository: https://github.com/repk/gxlimg/blob/master/README
-.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/board/amlogic/p212/README.libretech-cc
+.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/doc/board/amlogic/p212.rst