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 <giacomo.travaglini@arm.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>