cpu_freq: Add CPU Frequency Scaling subsystem

Add a CPU frequency scaling subsystem, allowing a policy
algorithm to control the frequency of a given SoC/MCU
automatically at runtime.

Implement a basic, "on-demand" policy algorithm which
iterates through the P-states supported by the SoC and
selects the first P-state where it's trigger threshold is
less than the CPU load.

The CPU frequency scaling subsystem does not currently
support SMP. The CPU load measurement can be made to support
SMP since statistics are measured from the scheduler.

Co-authored-by: Eric Hay <Eric.Hay@analog.com>
Signed-off-by: Sean Kyer <Sean.Kyer@analog.com>