ARM64: spinlock: Add SEV and dsb in unlock code

In certain unexplained cases, the stlr alone might not wakeup
the processor waiting in WFE on a spinlock.
Add an explicity dsb and SEV in write_unlock, read_unlock
and spin_unlock to ensure that the core waiting on the lock
wakes up from WFE.
Selectable by config option SEV_IN_LOCK_UNLOCK

Change-Id: I691ff5713d4d564623b75b053b40d1f46d74868a
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>