Make the simple-pm-bus driver probe the Layerscape SCFG dt nodes and
populate platform_device structures from its child dt nodes.
This is now needed because its child interrupt-controller - ls-extirq -
is being handled as a platform_device instead of being initialized
through the IRQCHIP_DECLARE infrastructure which impeded its parent IRQ
retrieval through the blamed commit.
Note that this does not set ONLY_BUS because that enables the
of_platform_populate() call. The extra power management operations which
are enabled by that are not required but harmless.