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: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>