NET: Add driver for Octeon MDIO buses.

The Octeon SOC has two types of Ethernet ports, each type with its own
driver.  However, the PHYs for all the ports are controlled by a
common MDIO bus.  Because the mdio driver is not associated with a
particular driver, but is instead a system level resource, we create s
stand-alone driver for it.

As for the driver, we put the register definitions in
arch/mips/include/asm/octeon where most of the other Octeon register
definitions live.  This is a platform driver with the platform device
for "mdio-octeon" being registered in the platform startup code.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>