ssb: provide phy address for Gigabit Ethernet driver

Add a function to provide the phy address which should be used to the
Gigabit Ethernet driver connected to ssb.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>