mv643xx_eth: convert to use the Marvell Orion MDIO driver

This patch converts the Marvell MV643XX ethernet driver to use the
Marvell Orion MDIO driver. As a result, PowerPC and ARM platforms
registering the Marvell MV643XX ethernet driver are also updated to
register a Marvell Orion MDIO driver. This driver voluntarily overlaps
with the Marvell Ethernet shared registers because it will use a subset
of this shared register (shared_base + 0x4 to shared_base + 0x84). The
Ethernet driver is also updated to look up for a PHY device using the
Orion MDIO bus driver.

For ARM and PowerPC we register a single instance of the "mvmdio" driver
in the system like it used to be done with the use of the "shared_smi"
platform_data cookie on ARM.

Note that it is safe to register the mvmdio driver only for the "ge00"
instance of the driver because this "ge00" interface is guaranteed to
always be explicitely registered by consumers of
arch/arm/plat-orion/common.c and other instances (ge01, ge10 and ge11)
were all pointing their shared_smi to ge00. For PowerPC the in-tree
Device Tree Source files mention only one MV643XX ethernet MAC instance
so the MDIO bus driver is registered only when id == 0.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>