arm64: defconfig: enable stmmac ethernet to defconfig

This patch enables the CONFIG_STMMAC_ETH to the default arm64 defconfig:

-CONFIG_STMMAC_ETH=m
+CONFIG_STMMAC_ETH=y
+CONFIG_DWMAC_IPQ806X=m
+CONFIG_DWMAC_MESON=m
+CONFIG_DWMAC_ROCKCHIP=m
+CONFIG_DWMAC_SUNXI=m
+CONFIG_DWMAC_SUN8I=m

The STMMAC ethernet controller is on the Stratix10 platform, and thus needs
driver to be in the kernel image for NFS to work.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>