ARM64: spinlock: Add SEV and dsb(ishst) 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(ishst) and SEV in write_unlock, read_unlock
and spin_unlock to ensure that the core waiting on the lock
wakes up from WFE.

ISHST variant of the DSB should be fine here since this would
be applicable for the inner shareable domain only with the
store instruction before it.

Selectable by config option ARM64_SEV_IN_LOCK_UNLOCK and it
is disabled by default.

CRs-Fixed: 962923
Change-Id: I691ff5713d4d564623b75b053b40d1f46d74868a
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>