Commit ad7061ed742e ("doc: Move device tree bindings documentation to
doc/device-tree-bindings") moved all device tree binding documentation
to doc/device-tree-bindings directory.

The current U-Boot project still have two documentation directories:

- doc/
- Documentation/

Move all documentation and sphinx files to doc directory so all content
can be in a common place.

Adapted to U-Boot v2019.04

Signed-off-by: Breno Lima
Acked-by: Ye Li