sim: ScopedMigration does nothing if both eqs are the same

sim: ScopedMigration does nothing if both eqs are the same

Added a check to avoid unlocking/locking the same event queue. Also,
added an optional parameter to enable the migration to be skipped. This
can be useful to disable the synchronization for certain runtime
conditions.

Change-Id: I4b03b3ffff4f9503153cd41dd8aa78705bf16cc4
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5730
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>