util: Add fastmodel in valid tag list
util: Add fastmodel in valid tag list

The "fastmodel" tag has been used since 2019-08-22 so it should be an
valid tag in commit header.

Change-Id: I0032deaabc94e5896851da9afc28e1b1a699fed3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23923
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5