while attaching cpus, if the FDT provides "capacity-dmips-mhz" track
the fastest set, and call cpu_topology_set() with slow=true for any
cpus that are not the fastest.
bug fix for cpu_topology_set(): actually set ci_is_slow for slow cpus.
with this change, and -current's recent scheduler changes, this means
that long running processes run on the faster cores. on RK3399 based
systems, i am seeing 20-50% speed ups for many tasks.
XXX: all this can be made common with armv7 big.little.
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