config: Add fastmodel cluster in fs_bigLITTLE.py

config: Add fastmodel cluster in fs_bigLITTLE.py

One can create a system with ARM FastModels CPU and GICv3 with
--cpu-type fastmodel --machine-type VExpressFastmodel options.
Currently the FastmodelCluster only supports one CPU.

Change-Id: I2e985f08f9df01a703e21441c6f9bc1fbae4a222
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20901
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>