rockchip: rk3288-vyasa: defconfig: Enable gmac support

Enable gmac support for rk3288-vyasa board.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>