mv643xx_eth: Define module alias for platform device

The mv643xx_eth driver can be loaded as a platform device, as is done by
various Orion (ARM) based devices.  The driver needs to define a module
alias for the platform driver so udev will load it automatically.
Tested with Debian on a QNAP TS-209.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>