sched: add arch_scale_min_freq_capacity to track minimum capacity caps

If the minimum capacity of a group is capped by userspace or internal
dependencies which are not otherwise visible to the scheduler, we need
a way to see these and integrate this information into the energy
calculations and task placement decisions we make.

Add arch_scale_min_freq_capacity to determine the lowest capacity which
a specific cpu can provide under the current set of known constraints.

Change-Id: Ied4a1dc0982bbf42cb5ea2f27201d4363db59705
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>