macb: support higher rate GEM MDIO clock divisors

GEM devices support larger clock divisors and have a different
range of divisors.  Program the MDIO clock divisors based on the
device type.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>