config: Clean up core timing model discovery

config: Clean up core timing model discovery

Instead of hard-coding timing models in CpuConfig.py, use
introspection to find them in the cores.arm model package.

Change-Id: I6642dc9cbc3f5beeeec748e716c9426c233d51ea
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3944
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>