csky: Move $(core-y) into arch/csky/Kbuild

Use the standard obj-y form to specify the sub-directories under
arch/csky/. Only leave core-y += arch/csky/$(CSKYABI)/ there.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Guo Ren <guoren@kernel.org>