Currently all omap_devices are forced to have the dummy device
'omap_device_parent' as a parent.  This was used to distinguish
omap_devices from "normal" platform_devices in the OMAP PM core code.

Now that we implement the PM core using PM domains, this is no longer
needed, and is removed.

This also frees up omap_devices to have a more complex parent/child
relationships that model actual device relationships.

The only in-tree user of omap_device_parent was the OMAP PM layer to
handle lost-context count for omap_devices.  That is now converted to
use the presence of the omap_device_pm_domain instead.
Signed-off-by: Kevin Hilman <khilman@ti.com>