MIPS: Fix "make clean" error due to recent changes

Commit 26bff9eb ("MIPS: Only include the platformfile needed")
moves platform-(CONFIG_XYZ) from arch/mips/xyz/Platform to arch/mips/
Kbuild.platforms. This change causes an error when "make clean":

  ./scripts/Makefile.clean:15: arch/mips/vr41xx/Makefile: No such file or directory
  make[3]: *** No rule to make target `arch/mips/vr41xx/Makefile'.  Stop.
  make[2]: *** [arch/mips/vr41xx] Error 2
  make[1]: *** [_clean_arch/mips] Error 2
  make: *** [sub-make] Error 2

Clean-files are defined in arch/mips/Kbuild:

  obj- := $(platform-)

Due to the movement of platform-(CONFIG_XYZ), "make clean" will enter
arch/mips/vr41xx/ whether CONFIG_MACH_VR41XX is defined or not. Because
there is no Makefile in arch/mips/vr41xx/, "make clean" fails. I don't
know what is the best way to fix it, but it seems like we can avoid this
error by changing the obj- definition:

  obj- := $(platform-y)

Fixes: 26bff9eb ("MIPS: Only include the platformfile needed")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>