fastmodel: Implement port proxies.

fastmodel: Implement port proxies.

This plumbing is simple and largely copied from other implementations
within gem5. This mechanism should be refactored so that the
duplication is unnecessary.

Change-Id: Ibcdf759b7fba1d574e8e2ba04249afdd92c6560c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22120
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>