net: dsa: ocelot: add support for external phys

The VSC7512 has four ports with internal phys that are already supported.
There are additional ports that can be configured to work with external
phys.

Add support for these additional ethernet ports.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>