dt-bindings: net: ethernet-controller: Add informative text about RGMII delays

Device Tree and Ethernet MAC driver writers often misunderstand RGMII
delays. Rewrite the Normative section in terms of the PCB, is the PCB
adding the 2ns delay. This meaning was previous implied by the
definition, but often wrongly interpreted due to the ambiguous wording
and looking at the definition from the wrong perspective. The new
definition concentrates clearly on the hardware, and should be less
ambiguous.

Add an Informative section to the end of the binding describing in
detail what the four RGMII delays mean. This expands on just the PCB
meaning, adding in the implications for the MAC and PHY.

Additionally, when the MAC or PHY needs to add a delay, which is
software configuration, describe how Linux does this, in the hope of
reducing errors. Make it clear other users of device tree binding may
implement the software configuration in other ways while still
conforming to the binding.

Fixes: 9d3de3c5 ("dt-bindings: net: Add YAML schemas for the generic Ethernet options")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250430-v6-15-rc3-net-rgmii-delays-v2-1-099ae651d5e5@lunn.ch


Signed-off-by: Jakub Kicinski <kuba@kernel.org>