rockchip: dwmmc: add handling for u-boot, spl-fifo-mode

Rockchips dwmmc controllers can't do dma to non-ddr addresses,
like for example the soc-internal sram but during boot parts of
TrustedFirmware need to be placed there from the read FIT image.

So add handling for a u-boot,spl-fifo-mode to not put the mmc
controllers into fifo mode for all time.

The regular fifo-mode property still takes precedent and only
if not set do we check for the spl-specific property.

Suggested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>