phy: add reverse MII PHY connection type

The PHY library currently does not know about the the reverse MII
connection type. Add it to the list of supported PHY modes and update
of_get_phy_mode() to support it and look for the string "rev-mii".

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>