ARM: sunxi: Enable GMAC in sunxi_defconfig

Since the support of the GMAC has been merged, we're using it as the ethernet
controller on the A20 devices.

However, sunxi_defconfig wasn't selecting it hence breaking the NFS boot.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/configs/sunxi_defconfig

@@ -37,7 +37,7 @@ CONFIG_SUN4I_EMAC=y
 # CONFIG_NET_VENDOR_NATSEMI is not set
 # CONFIG_NET_VENDOR_SEEQ is not set
 # CONFIG_NET_VENDOR_SMSC is not set
-# CONFIG_NET_VENDOR_STMICRO is not set
+CONFIG_STMMAC_ETH=y
 # CONFIG_NET_VENDOR_WIZNET is not set
 # CONFIG_WLAN is not set
 CONFIG_SERIAL_8250=y