cpufreq: dt: Refactor initialization to handle probe deferral properly

cpufreq-dt is currently unable to handle -EPROBE_DEFER properly
because the error code is not propagated for the cpufreq_driver->init()
callback. Instead, it attempts to avoid the situation by temporarily
requesting all resources within resources_available() and releasing them
again immediately after. This has several disadvantages:

  - Whenever we add something like interconnect handling to the OPP core
    we need to patch cpufreq-dt to request these resources early.

  - resources_available() is only run for CPU0, but other clusters may
    eventually depend on other resources that are not available yet.
    (See FIXME comment removed by this commit...)

  - All resources need to be looked up several times.

Now that the OPP core can propagate -EPROBE_DEFER during initialization,
it would be nice to avoid all that trouble and just propagate its error
code when necessary.

This commit refactors the cpufreq-dt driver to initialize private_data
before registering the cpufreq driver. We do this by iterating over
all possible CPUs and ensure that all resources are initialized:

  1. dev_pm_opp_get_opp_table() ensures the OPP table is allocated
     and initialized with clock and interconnects.

  2. dev_pm_opp_set_regulators() requests the regulators and assigns
     them to the OPP table.

  3. We call dev_pm_opp_of_get_sharing_cpus() early so that we only
     initialize the OPP table once for each shared policy.

With these changes, we actually end up saving a few lines of code,
the resources are no longer looked up multiple times and everything
should be much more robust.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
[ Viresh: Use list_head structure for maintaining the list and minor
	  changes ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>