cpufreq: dt: Add generic platform-device creation support

Multiple platforms are using the generic cpufreq-dt driver now, and all
of them are required to create a platform device with name "cpufreq-dt",
in order to get the cpufreq-dt probed.

Many of them do it from platform code, others have special drivers just
to do that.

It would be more sensible to do this at a generic place, where all such
platform can mark their entries.

This patch adds a separate file to get this device created. Currently
the compat list of platforms that we support is empty, and will be
filled in as and when we move platforms to use it.

It always compiles as part of the kernel and so doesn't need a
module-exit operation.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>