sched/fair: introduce an arch scaling function for max frequency capping

The max frequency scaling factor is defined as:

  max_freq_scale = policy_max_freq / cpuinfo_max_freq

To be able to scale the cpu capacity by this factor introduce a call to
the new arch scaling function arch_scale_max_freq_capacity() in
update_cpu_capacity() and provide a default implementation which returns
SCHED_CAPACITY_SCALE.

Another subsystem (e.g. cpufreq) can overwrite this default implementation,
exactly as for frequency and cpu invariance. It has to be enabled by the
arch by defining arch_scale_max_freq_capacity to the actual
implementation.

Change-Id: I266cd1f4c1c82f54b80063c36aa5f7662599dd28
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>