mv643xx_eth: Remove a stale PPC_MULTIPLATFORM

PPC_MULTIPLATFORM was killed in commit 28794d34 but this stale occurrence was
hiding the mv643xx_eth driver in some cases (e.g. Pegasos II)

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>