net: mv643xx_eth: Fetch the phy connection type from DT

The MAC is capable of RGMII mode and that is probably a more typical
connection type than GMII today (eg it is used by Marvell Reference
designs for several SOCs). Let DT users specify the standard

   phy-connection-type = "rgmii-id";

On a phy node.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>