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>

arch/arm/dts/rk3308-u-boot.dtsi

@@ -12,6 +12,8 @@
 };
 
 &emmc {
+	/* mmc to sram can't do dma, prevent aborts transferring TF-A parts */
+	u-boot,spl-fifo-mode;
 	u-boot,dm-pre-reloc;
 };