driver: ldpaa: Add api to return linked PHY ID of DPMAC

DPMAC represents physical line on the board. This physical
line eventually asscociate with on-board PHY.

So Add an api to return linked PHY ID of DPMAC object.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>