dt-bindings: phy: sparx5: document lan969x

Lan969x is going to reuse the existing Sparx5 SERDES driver - document
that by adding compatible strings for the different SKU's that we
support, and a short description of the SERDES types and data rates
supported.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240909-sparx5-lan969x-serdes-driver-v2-8-d695bcb57b84@microchip.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>