mmc: sdhci: add support for using external DMA devices

Some standard SD host controllers can support both external dma
controllers as well as ADMA/SDMA in which the SD host controller
acts as DMA master. TI's omap controller is the case as an example.

Currently the generic SDHCI code supports ADMA/SDMA integrated in
the host controller but does not have any support for external DMA
controllers implemented using dmaengine, meaning that custom code is
needed for any systems that use an external DMA controller with SDHCI.

Fixes by Faiz Abbas <faiz_abbas@ti.com>:
1. Map scatterlists before dmaengine_prep_slave_sg()
2. Use dma_async() functions inside of the send_command() path and call
terminate_sync() in non-atomic context in case of an error.

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/20200116105154.7685-4-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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