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/Makefile b/Makefile
index 9fba74d..6335834 100644
--- a/Makefile
+++ b/Makefile
@@ -168,7 +168,7 @@
 # Use 'make C=2' to enable checking of *all* source files, regardless
 # of whether they are re-compiled or not.
 #
-# See the file "Documentation/sparse.txt" for more details, including
+# See the file "doc/sparse.txt" for more details, including
 # where to get the "sparse" utility.
 
 ifeq ("$(origin C)", "command line")
@@ -1916,7 +1916,7 @@
 	@echo  '  coccicheck      - Execute static code analysis with Coccinelle'
 	@echo  ''
 	@echo  'Documentation targets:'
-	@$(MAKE) -f $(srctree)/Documentation/Makefile dochelp
+	@$(MAKE) -f $(srctree)/doc/Makefile dochelp
 	@echo  ''
 	@echo  '  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build'
 	@echo  '  make V=2   [targets] 2 => give reason for rebuild of target'
@@ -1945,7 +1945,7 @@
 	       linkcheckdocs dochelp refcheckdocs
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
-	$(Q)$(MAKE) $(build)=Documentation $@
+	$(Q)$(MAKE) $(build)=doc $@
 
 endif #ifeq ($(config-targets),1)
 endif #ifeq ($(mixed-targets),1)