net: stmmac: dwmac-qcom-ethqos: add support for the optional serdes phy

On sa8775p platforms, there's a SGMII SerDes PHY between the MAC and
external PHY that we need to enable and configure.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>