dt-bindings: net: document ethernet PHY package nodes

dt-bindings: net: document ethernet PHY package nodes

Document ethernet PHY package nodes used to describe PHY shipped in
bundle of 2-5 PHY. The special node describe a container of PHY that
share common properties. This is a generic schema and PHY package
should create specialized version with the required additional shared
properties.

Example are PHY packages that have some regs only in one PHY of the
package and will affect every other PHY in the package, for example
related to PHY interface mode calibration or global PHY mode selection.

The PHY package node MUST declare the base address used by the PHY driver
for global configuration by calculating the offsets of the global PHY
based on the base address of the PHY package.

Each reg of the PHYs defined in the PHY package node is absolute and
describe the real address of the Ethernet PHY on the bus.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>