net: davinci_emac: Fix ioremap for devices with MDIO within the EMAC address space

Some devices like dm816x have the MDIO registers within the first EMAC
instance address space. Let's fix the issue by allowing to pass an
optional second IO range for the EMAC control register area.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>