doc: Move device tree bindings documentation to doc/device-tree-bindings

Currently the U-Boot project contains 2 documentation directories:

- doc/
- Documentation/

The Documentation directory only contains device tree bindings related
content, so move the 3 files to doc/device-tree-bindings/.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
3 files changed