sim: Add System method for MasterID lookup

sim: Add System method for MasterID lookup

A new method (lookupMasterId) has been added to the System. A client
should use it when querying the System for the MasterID of a particular
master. It changes from getMasterId since it is
not registering a new MasterID if the master is not found in the
master's list.

Change-Id: I701158d22e235085bba9ab91154fbb702cae1467
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11969
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>