arm: Frequency invariant scheduler load-tracking support

Implements arch-specific function to provide the scheduler with a
frequency scaling correction factor for more accurate load-tracking. The
factor is:

	current_freq(cpu) * SCHED_CAPACITY_SCALE / max_freq(cpu)

This implementation only provides frequency invariance. No
micro-architecture invariance yet.

Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>