staging: board: Migrate away from __pm_genpd_name_add_device()

The named genpd APIs are deprecated. Hence convert the board staging
code from using genpd names to DT node paths.

For now this supports PM domains with "#power-domain-cells = <0>" only.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>