arch-mips: added missing override specifier (o3)

Change-Id: Ic538825a2964fd62def672b933a83067a15bd12a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17648
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>