net: phy: adin: add support for Analog Devices PHYs

This change adds support for Analog Devices Industrial Ethernet PHYs.
Particularly the PHYs this driver adds support for:
 * ADIN1200 - Robust, Industrial, Low Power 10/100 Ethernet PHY
 * ADIN1300 - Robust, Industrial, Low Latency 10/100/1000 Gigabit
   Ethernet PHY

The 2 chips are register compatible with one another. The main difference
being that ADIN1200 doesn't operate in gigabit mode.

The chips can be operated by the Generic PHY driver as well via the
standard IEEE PHY registers (0x0000 - 0x000F) which are supported by the
kernel as well. This assumes that configuration of the PHY has been done
completely in HW, according to spec.

Configuration can also be done via registers, which will be supported by
this driver.

Datasheets:
  https://www.analog.com/media/en/technical-documentation/data-sheets/ADIN1300.pdf
  https://www.analog.com/media/en/technical-documentation/data-sheets/ADIN1200.pdf



Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>