sunxi: Enable EMAC on the Cubietruck Plus

The Cubietruck Plus has an RTL8211E PHY connected to the EMAC using
RGMII. The PHY is powered by DLDO4 @ 3.3V, while the I/O pins are
powered by DLDO3 @ 2.5V.

This patch enables the EMAC and Realtek PHY drivers in the defconfig.
The device tree file already has the EMAC enabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>