ARM: 8098/1: mcs lock: implement wfe-based polling for MCS locking

This patch introduces a wfe-based polling loop for spinning on contended
MCS locks and waking up corresponding waiters when the lock is released.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>