Add a structure holding required operations for each device such as device
initialization, PHY port read or write, a checker whether PHY interface is
supported on a certain port, MAC port setup for either bus pad or a
specific PHY interface.

The patch is done for ready adding a new hardware MT7531, and keep the
same setup logic of existing hardware.
Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>