That error can happen if you give
git a path that isn't a git repo. According to the homepage:
To clone one of these trees using Git type:
git clone git://git.denx.de/<project_name> <your_repository_name>/
Cloning per HTTP is also supported (just replace git:// with http:// in the examples above). Note that native git transfer is much faster and more robust. Use HTTP only if you have no other choice.
And the project listed on that page is named "u-boot.git". So you want to do one of these:
git clone git://git.denx.de/u-boot.git
git clone http://git.denx.de/u-boot.git