mmc: sdhci: Allow disabling of SDMA in SPL

Rockchip emmc devices have a similar issue to Rockchip dwmmc devices,
where performing DMA to SRAM later causes issues with suspend/resume.

Allow us to toggle SDMA in SPL for sdhci similar to ADMA support, so we
can ensure DMA is not used when loading the SRAM code.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
[jonas@kwiboo.se: add Kconfig default value and fix ADMA typo]
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>