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

Documentation/devicetree/bindings/mmc/sdhci-omap.txt

@@ -15,6 +15,13 @@ Required properties:
 		 "hs200_1_8v",
 - pinctrl-<n> : Pinctrl states as described in bindings/pinctrl/pinctrl-bindings.txt
 
+Optional properties:
+- dmas:		List of DMA specifiers with the controller specific format as described
+		in the generic DMA client binding. A tx and rx specifier is required.
+- dma-names:	List of DMA request names. These strings correspond 1:1 with the
+		DMA specifiers listed in dmas. The string naming is to be "tx"
+		and "rx" for TX and RX DMA requests, respectively.
+
 Example:
 	mmc1: mmc@4809c000 {
 		compatible = "ti,dra7-sdhci";
@@ -22,4 +29,6 @@ Example:
 		ti,hwmods = "mmc1";
 		bus-width = <4>;
 		vmmc-supply = <&vmmc>; /* phandle to regulator node */
+		dmas = <&sdma 61 &sdma 62>;
+		dma-names = "tx", "rx";
 	};