ARM: dts: gemini: ns2502: permit to use gigabit

I believed that gigabit was not working due to some unknown missing GPIO.
In fact, gigabit worked when REALTEK_PHY was compiled out.
So the problem was due to PHY delay and we need to use rgmii-id.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>