csky: Cleanup asm/spinlock.h

There are two implementation of spinlock in arch/csky:
 - simple one (NR_CPU = 1,2)
 - tick's one (NR_CPU = 3,4)
Remove the simple one.

There is already smp_mb in spinlock, so remove the definition of
smp_mb__after_spinlock.

Link: https://lore.kernel.org/linux-csky/20200807081253.GD2674@hirez.programming.kicks-ass.net/#t


Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Peter Zijlstra <peterz@infradead.org>k
Cc: Arnd Bergmann <arnd@arndb.de>