config: Remove support for CPU aliases.

config: Remove support for CPU aliases.

This was added for backwards compatability, but it adds a decent amount
of complexity.

The table below shows what CPU class name to use in place of a given
alias.

+==========+========================================================+
|  Alias   |                       CPU class                        |
+==========+========================================================+
| timing   | TimingSimpleCPU                                        |
| atomic   | AtomicSimpleCPU                                        |
| minor    | MinorCPU                                               |
| detailed | DrivO3CPU                                              |
| kvm      | ArmKvmCPU, ArmV8KvmCPU or X86KvmCPU, depending on arch |
| trace    | TraceCPU                                               |
+==========+========================================================+

Change-Id: I251c4f64b7869c6b64dd25b36967ae240f01ef08
Reviewed-on: https://gem5-review.googlesource.com/2940
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>