phylib: cleanup marvell.c a bit

Simplify the marvell driver init a bit: Make the supported devices an
array instead of explicitly registering each structure. This makes it
considerably easier to add new devices down the road.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>