ibm_newemac: Add MII mode support to the EMAC RGMII bridge.

This patch adds support to the RGMII handler in the EMAC driver for
the MII PHY mode such that device tree entries of the form `phy-mode = "mii";'
are recognized and handled appropriately.

While logically, in software, "gmii" and "mii" modes are the same,
they are wired differently, so it makes sense to allow DTS authors to
specify each explicitly.

Signed-off-by: Grant Erickson <gerickson@nuovations.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>