dt-bindings: sdhci-omap: Add properties for using external dma

dt-bindings: sdhci-omap: Add properties for using external dma

sdhci-omap can support both external dma controller via dmaengine
framework as well as ADMA which standard SD host controller
provides. Add binding documentation for these external dma properties.

Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200116105154.7685-2-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>