net: ftgmac100: Add support for board specific PHY interface address

ftgmac100 driver is using hard-coded PHY interface address of zero.
Each board can have different PHY interface address (phy_addr).
This commit modifies the driver to make use of board specific address
by leveraging CONFIG_PHY_ADDR.

Signed-off-by: Thirupathaiah Annapureddy <thiruan@linux.microsoft.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>