Makefile: fix ctags/etags clean targets

Commit efcf861 (kbuild: use scripts/Makefile.clean)
refactored the cleaning targets and accidentially replaced the actually
generated "ctags" and "etags" files in the file list by "tags" and "TAGS".
"tags" and "TAGS" are not part of the Makefile build targets and
therefore should not be a part of the list for clean targets.

Substitute the actually generated files instead, to fix the clean
targets behavior.

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>

Makefile

@@ -1222,7 +1222,7 @@ CLOBBER_FILES += u-boot* MLO* SPL System.map
 MRPROPER_DIRS  += include/config include/generated          \
 		  .tmp_objdiff
 MRPROPER_FILES += .config .config.old \
-		  tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
+		  ctags etags cscope* GPATH GTAGS GRTAGS GSYMS \
 		  include/config.h include/config.mk
 
 # clean - Delete most, but leave enough to build external modules