dt-bindings: phy: renesas: Document Renesas Ethernet SERDES

Document Renesas Etherent SERDES for R-Car S4-8 (r8a779f0).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221108005500.3011449-2-yoshihiro.shimoda.uh@renesas.com


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