net/sb1250: setup the pdevice within the soc code

doing it within the driver does not look good.
And surely isn't how platform devices were meat to be used.

Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>