mv643xx_eth: Place explicit port number in mv643xx_eth_platform_data

We were using the platform_device.id field to identify which ethernet
port is used for mv643xx_eth device.  This is not generally correct.
It will be incorrect, for example, if a hardware platform uses a single
port but not the first port.  Here, we add an explicit port_number field
to struct mv643xx_eth_platform_data.

This makes the mv643xx_eth_platform_data structure required, but that
isn't an issue since all users currently provide it already.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>