dt-bindings: net: ti: k3-am654-cpsw-nuss: Document Serdes PHY

Update bindings to include Serdes PHY as an optional PHY, in addition to
the existing CPSW MAC's PHY. The CPSW MAC's PHY is required while the
Serdes PHY is optional. The Serdes PHY handle has to be provided only
when the Serdes is being configured in a Single-Link protocol. Using the
name "serdes-phy" to represent the Serdes PHY handle, the am65-cpsw-nuss
driver can obtain the Serdes PHY and request the Serdes to be
configured.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>