PM / OPP: move cpufreq specific OPP functions out of generic OPP library

CPUFREQ specific functions for OPP (Operating Performance Points) can
be isolated to just cpufreq. This allows for independent modifications
as needed. The functionality desired by cpufreq can easily be provided
by existing functions and any future "special handling" needed for
cpufreq drivers can similarly be handled.

With this change the internal storage order of OPP entries are no
longer a limiting factor for how we need cpufreq table to look like.

Signed-off-by: Nishanth Menon <nm@ti.com>