net: stmmac: dwmac-qcom-ethqos: add missing include

device_get_phy_mode() is declared in linux/property.h but this header
is not included.

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