dt-bindings: phy: lynx-28g: add compatible strings per SerDes and instantiation

The 28G Lynx SerDes is instantiated 3 times in the NXP LX2160A SoC and
twice in the NXP LX2162A. All these instances share the same register
map, but the number of lanes and the protocols supported by each lane
differs in a way that isn't detectable by the programming model.

For example, not all lanes of all SerDes block instantiations support
25GbE.

So, using a generic "fsl,lynx-28g" compatible string and expecting all
SerDes instantiations to use it was a mistake that needs to be fixed.

The option chosen is to encode the SoC and the SerDes instance in the
compatible string, with everything else being the responsibility of the
driver to derive.

An alternative considered but dismissed was to add sufficient device
tree properties to describe the per-lane differences (implying:
supported protocols), as well as the different lane count.

Any decision made for the 28G Lynx should be consistent with the
decisions taken for the yet-to-be-introduced 10G Lynx SerDes (older
generation for older SoCs), because of how similar they are.

I've seen the alternative at play in this unmerged patch set for the 10G
Lynx here, and I didn't like it:
https://lore.kernel.org/linux-phy/20230413160607.4128315-3-sean.anderson@seco.com/

This is because there, we have a higher degree of variability in the
PCCR register values that need to be written per protocol. This makes
that approach more drawn-out and more prone to errors, compared to the
compatible strings which are more succinct and obviously correct.

NXP SoC reference manuals clearly document the SerDes instantiations as
not identical, and refers to them as such (SerDes 1, 2, etc).

The per-SoC compatible string is prepended to the "fsl,lynx-28g" generic
compatible, which is left there for compatibility with old kernels. An
exception would be LX2160A SerDes #3, which at the time of writing is
not described in fsl-lx2160a.dtsi. As "fsl,lx2160a-serdes3" implies it
is a 28G Lynx SerDes, it makes "fsl,lynx-28g" redundant so we don't
accept it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Tested-by: Josua Mayer <josua@solid-run.com>
Link: https://patch.msgid.link/20260511150023.1903577-2-vladimir.oltean@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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