csky: Remove custom asm/atomic.h implementation

Use generic atomic implementation based on cmpxchg. So remove csky
asm/atomic.h.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>