cpufreq/sched: Use cpu max freq rather than policy max

When we convert capacity into frequency, we used policy->max to get
the max freq of the cpu. Since this can be changed by userspace policy
or thermal events, we are potentially asking for a lower frequency
than the utilization demands.

Change over to using cpuinfo.max which is the max freq supported by
that cpu rather than the currently-chosen max. Frequency granted still
honours the max policy.

Tested by setting a userspace policy and observing the relevant vars
in a trace. In this instance, we ask for around 1ghz instead of 620MHz.

freq_new=1013512
unfixed_freq_new=624487
capacity=546
cpuinfo_max=1900800
policy_max=1171200

Change-Id: I8c5694db42243c6fb78bb9be9046b06ac81295e7
Signed-off-by: Chris Redpath <chris.redpath@arm.com>