csky: Minimize defconfig to support buildroot config.fragment

Some bsp (eg: buildroot) has defconfig.fragment design to add more
configs into the defconfig in linux source code tree. For example,
we could put different cpu configs into different defconfig.fragments,
but they all use the same defconfig in Linux.

Signed-off-by: Ma Jun <majun258@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>