doc: Remove duplicated documentation directory

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.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 9046084..bea3122 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -476,7 +476,7 @@
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-efi.git
 F:	doc/README.uefi
 F:	doc/README.iscsi
-F:	Documentation/efi.rst
+F:	doc/efi.rst
 F:	include/capitalization.h
 F:	include/charset.h
 F:	include/cp1250.h