m68k: Use CLEAN_FILES to clean up files

The log of 'make ARCH=m68k clean' does not look nice.

$ make ARCH=m68k clean
  CLEAN   arch/m68k/kernel
  [ snip ]
  CLEAN   usr
rm -f vmlinux.gz vmlinux.bz2
  CLEAN   vmlinux.symvers modules.builtin modules.builtin.modinfo

Use CLEAN_FILES to simplify the code, and beautify the log.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200617031153.85858-1-masahiroy@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>