doc: Fix broken external link for Odroid C2

The file README.odroid-c2 has been moved in the U-Boot repository.

Reference the official uplink repository.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Change-Id: Ie72c7aefd6363a406f88ad2c87faee1c7a2125a3

docs/plat/meson-gxbb.rst

@@ -23,4 +23,4 @@ This port has been tested in a ODROID-C2. After building it, follow the
 instructions in the `U-Boot repository`_, replacing the mentioned **bl31.bin**
 by the one built from this port.
 
-.. _U-Boot repository: https://github.com/u-boot/u-boot/blob/master/board/amlogic/odroid-c2/README.odroid-c2
+.. _U-Boot repository: https://gitlab.denx.de/u-boot/u-boot/-/blob/master/board/amlogic/p200/README.odroid-c2