sched/fair: introduce minimum capacity capping sched feature

We have the ability to track minimum capacity forced onto a CPU by
userspace or external actors. This is provided though a minimum
frequency scale factor exposed by arch_scale_min_freq_capacity.

The use of this information is enabled through the MIN_CAPACITY_CAPPING
feature. If not enabled, the minimum frequency scale factor will
remain 0 and it will not impact energy estimation or scheduling
decisions.

Change-Id: Ibc61f2bf4fddf186695b72b262e602a6e8bfde37
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>