net: mvmdio: allow platform device style registration

This patch changes the mvmdio driver not to use device tree
helper functions such as of_mdiobus_register() and of_iomap() so we can
instantiate this driver using a classic platform_device approach. Use
the device manager helper to ioremap() the base register cookie so we
get automatic freeing upon error and removal. This change is harmless
for Device Tree platforms because they will get the driver be registered
the same way as it was before.

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