fsl/fman: add API to get the device behind a fman port

Add an API that retrieves the 'struct device' that the specified FMan
port probed against. The new API will be used in a subsequent patch
that corrects the DMA devices used by the dpaa_eth driver.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>