[PATCH] mv643xx_eth: restrict to 32-bit PPC_MULTIPLATFORM

No 64-bit PPC_MULTIPLATFORM platforms use the mv643xx_eth driver,
so build it only on PPC32.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Acked-by: Sven Luther <sl@bplan-gmbh.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>