[POWERPC] Remove generated files on make clean

vmlinux.lds and dtc-parser.tab.h get created but never cleaned up.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 0662ae4..c1baf9d 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -104,3 +104,5 @@
 PHONY += systbl_chk
 systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
 	$(call cmd,systbl_chk)
+
+clean-files := vmlinux.lds