net: phy: add MoCA PHY type

Some Ethernet MACs are connected to a MoCA PHY which will handle the
low-level job of sending Ethernet frames on the coaxial cable, these
Ethernet MACs need to know about it to be properly configured.
Add a new PHY mode "moca" and update the Device Tree parsing logic to
look for it.

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