arm64: dts: rockchip: enable eMMC for rk3399 EVB

Rockchip's rk3399 evaluation board has eMMC. Let's enable the
newly-added nodes.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>