fastmodel: Add a gem5Cpu attribute to the CortexA76x1.

fastmodel: Add a gem5Cpu attribute to the CortexA76x1.

This attribute is to let the fast model EVS CPU find and talk to the
gem5 CPU in case it needs a pointer to one of its ThreadContexts for
instance.

Also move the code that finds the clock period attribute/event to the
constructor. gem5 guarantees that the EVS is constructed before its
pointer is passed to the iris CPU wrapper, and so the EVS will have
had a chance to install those controls if it's going to.

Change-Id: I389ef0ba0f9d528140f40444baa5091a9ec338cd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21045
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>