The device managed resources are ran if the device has bus, which is not
the case of a root_device.

Let's use a platform_device instead.
Reviewed-by: NJavier Martinez Canillas <javierm@redhat.com>
Reviewed-by: NMaíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-8-4615a663a84a@cerno.techSigned-off-by: NMaxime Ripard <maxime@cerno.tech>