mv643xx_eth: smi sharing is a per-unit property, not a per-port one

Which top-level unit's SMI interface to use should be a property of
the top-level unit, not of the individual ports.  This patch moves the
->shared_smi pointer from the per-port platform data to the global
platform data.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>