sched/semaphore/spinlock.c: Add memory barrier operations in spin_unlock()

In ARM document regarding memory barrires, SP_DMB() must be issued
before changing a spinlock state to SP_UNLOCKED. However, we found
that SP_DSB() is also needed to ensure that spin_unlock() works
correctly for network streaming aging test.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>