mfd: Only unregister platform devices allocated by the mfd core

commit b9fbb62eb61452d728c39b2e5020739c575aac53 upstream.

mfd_remove_devices would iterate over all devices sharing a parent with
an mfd device regardless of whether they were allocated by the mfd core
or not. This especially caused problems when the device structure was
not contained within a platform_device, because to_platform_device is
used on each device pointer.

This patch defines a device_type for mfd devices and checks this is
present from mfd_remove_devices_fn before processing the device.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Tested-by: Peter Tyser <ptyser@xes-inc.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>