mv643xx_eth: get rid of static variables, allow multiple instances

Move mv643xx_eth's static state (ethernet register block base address
and MII management interface spinlock) into a struct hanging off the
shared platform device.  This is necessary to support chips that
contain multiple mv643xx_eth silicon blocks.

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