octeon: Move the Cavium driver

Move the Cavium driver to drivers/net/ethernet/octeon/ and
make the necessary Kconfig and Makefile changes.

CC: David Daney <david.daney@cavium.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: David Daney <david.daney@cavium.com>