arm64: dts: qcom: sa8775p: add the SGMII PHY node

Add the internal SGMII/SerDes PHY node for sa8775p platforms.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Tested-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20230622120142.218055-2-brgl@bgdev.pl


Signed-off-by: Bjorn Andersson <andersson@kernel.org>