pmdomain: core: Add the genpd->dev to the genpd provider bus

To take the next step for a more common handling of the genpd providers,
let's add the genpd->dev to the genpd provider bus when registering a genpd
OF provider.

Also note, to allow us to add devices to the genpd provider bus we need to
make sure the bus has been registered first, which is done via a
core_initcall. Hence, calls to of_genpd_add_provider_simple|onecell() must
be done after the bus has been registered, else they will fail.

Suggested-by: Saravana Kannan <saravanak@google.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Tested-by: Hiago De Franco <hiago.franco@toradex.com> # Colibri iMX8X
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> # TI AM62A,Xilinx ZynqMP ZCU106
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-7-ulf.hansson@linaro.org