PM / OPP: Move CONFIG_OF dependent code in a separate file

Recently, a few issues were noticed in the code where CONFIG_OF wasn't
consistently used for many routines. The core file is big enough now and
ifdef hackery makes it less readable.

Move OF-specific code to another file and compile that only if CONFIG_OF
is enabled.

Compile-tested:
- For ARM (exynos) with CONFIG_OF enabled
- For X86 with CONFIG_OF disabled (have to enable CONFIG_PM_OPP separately)

No functional changes.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>