net: stmmac: dwmac-qcom-ethqos: add support for emac4 on sa8775p platforms

sa8775p uses EMAC version 4, add the relevant defines, rename the
has_emac3 switch to has_emac_ge_3 (has emac greater-or-equal than 3)
and add the new compatible.

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