arm: enable max frequency capping

Defines arch_scale_max_freq_capacity() to use cpufreq implementation.

Change-Id: Ibb5e3e04558477862565680e92ed265d8dee743e
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>