arm64: defconfig: enable stmmac and realtek PHY as modules

Some Amlogic GXBB boards use the stmmac ethernet driver.  Also, enable
the realtek PHY used on meson-gxbb-odroidc2.  The micrel PHY used on the
meson-gxbb-p20x boards is already enabled.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>