rockchip: rk3308: enable spl-fifo-mode for emmc

rockchip: rk3308: enable spl-fifo-mode for emmc

We need load some parts of ATF to sram, but rockchip
dwmmc controllers can't do dma to non-ddr addresses
space, so set the mmc controller into fifo mode in spl.

And show my best respect to Heiko's work for this solution.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>