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/README b/README
index b962ba7..a565748 100644
--- a/README
+++ b/README
@@ -58,8 +58,8 @@
 =========================
 
 The U-Boot source code is maintained in the Git repository at
-https://gitlab.denx.de/u-boot/u-boot.git ; you can browse it online at
-https://gitlab.denx.de/u-boot/u-boot
+https://source.denx.de/u-boot/u-boot.git ; you can browse it online at
+https://source.denx.de/u-boot/u-boot
 
 The "Tags" links on this page allow you to download tarballs of
 any version you might be interested in. Official releases are also