soc/qman: Don't add a new platform device for dma mapping

The qman portals are platform devices themselves, so they should
handle dma mappings.  Creating a dummy platform device in order to
support dma mapping operations is not justified (and not portable).
Instead, do the mapping against the first portal that has been
initialised.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Scott Wood <oss@buserror.net>