mmc: sdhci-omap: Add using external dma

sdhci-omap can support both external dma controller via dmaengine framework
as well as ADMA which standard SD host controller provides.

Fixes by Faiz Abbas <fazi_abbas@ti.com>:
1. Switch to DMA slave mode when using external DMA
2. Add offset to mapbase

Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20200116105154.7685-5-faiz_abbas@ti.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>