arm64: Enable frequency invariant scheduler load-tracking support

Defines arch_scale_freq_capacity() to use cpufreq implementation.

Including <linux/cpufreq.h> in topology.h like for the arm arch doesn't
work because of CONFIG_COMPAT=y (Kernel support for 32-bit EL0).
That's why cpufreq_scale_freq_capacity() has to be declared extern in
topology.h.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>