dev, arm: Fix multi-core KVM race in the generic timer

dev, arm: Fix multi-core KVM race in the generic timer

The generic timer sometimes needs to access global state. This can
lead to race conditions when simulating a multi-core KVM system where
each core lives in its own thread. In that case, the setMiscReg and
readMiscReg methods are called from the thread owning the CPU and not
the global device thread.

Change-Id: Ie3e982258648c8562cce0b30a0c122dfbfaf42cd
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2460
Reviewed-by: Weiping Liao <weipingliao@google.com>