rockchip: firefly-rk3399: enable dwmmc driver for the board

Enable mmc_dw_rockchip driver, disable CONFIG_SPL_OF_PLATDATA
first for some dependence patches still not merged.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Simon Glass <sjg@chromium.org>