net: phy: Add Cortina CS4340 driver

Add basic support for Cortina PHY drivers. Support only CS4340 for now.
The phys are not compatible with IEEE 802.3 clause 22/45 registers.

Implement proper read_status support. The generic 10G phy driver causes
bus register access errors.

The driver should be described using the "ethernet-phy-id" device tree
compatible.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>