Correct U-Boot upstream repository

The U-Boot source moves to https://source.denx.de/u-boot/u-boot.git
effective 2021-02-28.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/board/ti/j721e/README b/board/ti/j721e/README
index 757a59c..c33afa4 100644
--- a/board/ti/j721e/README
+++ b/board/ti/j721e/README
@@ -132,7 +132,7 @@
 	Branch: master
 
 4. U-Boot:
-	Tree: https://gitlab.denx.de/u-boot/u-boot
+	Tree: https://source.denx.de/u-boot/u-boot
 	Branch: master
 
 Build procedure: