kbuild: fix bugs in cleaning targets

"make clean", "make clobber", "make mrproper" and "make distclean"
missed to clean-up some files when they were run with
O=<some_dir> option.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Wolfgang Denk <wd@denx.de>
diff --git a/dts/Makefile b/dts/Makefile
index 9907463f..e59550c 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -44,4 +44,4 @@
 clean-files := dt.dtb.S
 
 # Let clean descend into dts directories
-subdir- += ../arch/*/dts
+subdir- += ../arch/arm/dts ../arch/microblaze/dts ../arch/sandbox/dts ../arch/x86/dts