rockchip: rk3188: add u-boot-specific mmc properties

The dwmmc controllers on rk3188 do not have idma support, so need to
use the fifo-mode and it my tests they became confused and stopped
working if the frequency was to high.

While I only tested in somewhat bigger steps, 32MHz for example
hung the controller, while reducing it to 16MHz worked just fine
and is reasonably fast to load a kernel from mmc.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>