PM / Domains: Export of_genpd_get_from_provider function

This function looks up a PM domain form the provider. This will be
useful to add parent/child domain relationship from the SoC specific
code. The caller of the function must make sure that PM domain provider
is already registered.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>