rockchip: firefly-rk3399: dts: enable sdmmc device

Enable sdmmc device and add the spl boot device sequence.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>