cpu-o3: Make the smtROBPolicy a Param.ScopedEnum

cpu-o3: Make the smtROBPolicy a Param.ScopedEnum

The smtROBPolicy is a parameter in the o3 cpu that can have 3
different values. Previously this setting was done through a string
and a parser function would turn it into a c++ enum value. This
changeset turns the string into a python Param.ScopedEnum.

Change-Id: Ie104d055dbbc6e44997ae0c1470de714239be5a3
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15399
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>