powerpc/dts: Makefile changes to clean and build dts

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/dts/Makefile b/dts/Makefile
index 36dfbe7..9a9a3d5 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -61,4 +61,4 @@
 clean-files := dt.dtb.S dt-spl.dtb.S
 
 # Let clean descend into dts directories
-subdir- += ../arch/arm/dts ../arch/microblaze/dts ../arch/mips/dts ../arch/sandbox/dts ../arch/x86/dts
+subdir- += ../arch/arm/dts ../arch/microblaze/dts ../arch/mips/dts ../arch/sandbox/dts ../arch/x86/dts ../arch/powerpc/dts