net: stmmac: dwmac-qcom-ethqos: add support for the phyaux clock

On sa8775p, the EMAC revision is 4 and we use SGMII instead of RGMII.
There's no "rgmii" clock but there's a fourth clock under a different
name: "phyaux". Add a new field to the chip data struct that specifies
the link clock name. Default to "rgmii" for backward compatibility.

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