ARM: mvebu: extend PMSU code to support dynamic frequency scaling

This commit adds the necessary code in the Marvell EBU PMSU driver to
support dynamic frequency scaling. In essence, what this new code does
is that it:

* registers the frequency operating points supported by the CPU;

* registers a clock notifier of the CPU clocks. The notifier function
listens to the newly introduced APPLY_RATE_CHANGE event, and uses
that to finalize the frequency transition by doing the part of the
procedure that involves the PMSU;

* registers a platform device for the cpufreq-generic driver, which
will take care of the CPU frequency transitions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1404920715-19834-3-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>