config: Discover CPU timing models based on target ISA

config: Discover CPU timing models based on target ISA

The CpuConfig helper currently assumes that all timing models live in
the cores.arm package. This ignores the potential mismatch between the
target ISA and the ISA assumptions made by the timing models.

Instead of unconditionally listing all CPU models in cores.arm, list
timing models from cores.generic and cores.${TARGET_ISA}. This ensures
that the listed timing models support the ISA that gem5 is targeting.

Change-Id: If6235af2118889638f56ac4151003f38edfe9485
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3947
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>