Disable SPL_MMC_SUPPORT if ROCKCHIP_SPL_BACK_TO_BROM is enabled.

Default SPL_MMC_SUPPORT to false when ROCKCHIP_SPL_BACK_TO_BROM is enabled.

Acked-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Signed-off-by: Sandy Patterson <apatterson@sightlogix.com>
Acked-by: Simon Glass <sjg@chromium.org>