mv643xx_eth: add PHY-less mode

On some boards, the mv643xx_eth MAC isn't connected to a PHY but
directly (via the MII/GMII/RGMII interface) to another MAC-layer
device.  This patch allows specifying ->phy_addr = -1 to skip all
PHY-related initialisation and run-time poking in that case.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>