csky: Set regs->usp to kernel sp, when the exception is from kernel

In the past, we didn't care about kernel sp when saving pt_reg. But in some
cases, we still need pt_reg->usp to represent the kernel stack before enter
exception.

For cmpxhg in atomic.S, we need save and restore usp for above.

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