net: stmmac: dwmac-qcom-ethqos: add support for SGMII

On sa8775p the MAC is connected to the external PHY over SGMII so add
support for it to the driver.

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