net: stmmac: dwmac-qcom-ethqos: prepare the driver for more PHY modes

In preparation for supporting SGMII, let's make the code a bit more
generic. Add a new callback for MAC configuration so that we can assign
a different variant of it in the future.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>