[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/boot/Makefile b/arch/powerpc/boot/Makefile
index 49797a4..63d07cc 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -147,6 +147,8 @@
 targets += dtc-src/dtc-parser.tab.c
 targets += dtc-src/dtc-lexer.lex.c
 
+clean-files += dtc-src/dtc-parser.tab.h
+
 ifdef DTC_GENPARSER
 BISON = bison
 FLEX = flex