[HACK] spinlock: send explicit event after store release instruction

stlr instruction in the spin_unlock function is expected to send an
implicit event to all CPU cores. In a cross chip scenario, FVP has
stability issues while sending this implicit event across chips.
To avoid the stability issues, add sev instruction which sends explicit
events to all cores. Explicit events are propagated across chips without
any stability issues.

Change-Id: I94db4f940e98f9b60f4120d0bde4930f9d31440c
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>