mv643xx_eth: inter-mv643xx SMI port sharing

There exist chips with up to four mv643xx_eth silicon blocks but
only one external SMI (MII management) interface -- the SMI logic
of the first block is shared by all the blocks.

Handle this by allowing a per-port override of which
mv643xx_eth_shared's SMI registers (and spinlock) to use.

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