ARM: 8540/1: decompressor: use clean-files instead of extra-y to clean files

This code works fine here, but it is tricky to use "extra-y" for
specifying files to be removed during "make clean".  Kbuild provides
"clean-files" for this purpose.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/boot/compressed/Makefile

@@ -94,8 +94,7 @@ targets       := vmlinux vmlinux.lds \
 		 lib1funcs.o ashldi3.o bswapsdi2.o \
 		 head.o $(OBJS)
 
-# Make sure files are removed during clean
-extra-y       += piggy.gzip piggy.lzo piggy.lzma piggy.xzkern piggy.lz4 \
+clean-files += piggy.gzip piggy.lzo piggy.lzma piggy.xzkern piggy.lz4 \
 		 lib1funcs.S ashldi3.S bswapsdi2.S $(libfdt) $(libfdt_hdrs) \
 		 hyp-stub.S