staging: board: Remove calls to of_genpd_get_from_provider()

Update the staging/board PM domain code to use the
of_genpd_add_subdomain() and remove any calls to
of_genpd_get_from_provider().

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>