pmdomain: core: Prevent registering devices before the bus

We must not register a consumer device to the genpd bus, before registering
the bus itself. Even if this doesn't seem to be an issue, let's add a
simple check to make sure we really avoid this from happening.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-5-ulf.hansson@linaro.org