dt-bindings: mmc: Add binding for BlueField SoC

This commit adds "mellanox,bluefield-dw-mshc" for dwmmc driver
extension on Mellanox BlueField SoC platform.
Signed-off-by: Liming Sun <lsun@mellanox.com>
Reviewed-by: David Woods <dwoods@mellanox.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/bluefield-dw-mshc.txt

+* Mellanox Bluefield SoC specific extensions to the Synopsys Designware
+  Mobile Storage Host Controller
+
+Read synopsys-dw-mshc.txt for more details
+
+The Synopsys designware mobile storage host controller is used to interface
+a SoC with storage medium such as eMMC or SD/MMC cards. This file documents
+differences between the core Synopsys dw mshc controller properties described
+by synopsys-dw-mshc.txt and the properties used by the Mellanox Bluefield SoC
+specific extensions to the Synopsys Designware Mobile Storage Host Controller.
+
+Required Properties:
+
+* compatible: should be one of the following.
+  - "mellanox,bluefield-dw-mshc": for controllers with Mellanox Bluefield SoC
+    specific extensions.
+
+Example:
+
+	/* Mellanox Bluefield SoC MMC */
+	mmc@6008000 {
+		compatible = "mellanox,bluefield-dw-mshc";
+		reg = <0x6008000 0x400>;
+		interrupts = <32>;
+		fifo-depth = <0x100>;
+		clock-frequency = <24000000>;
+		bus-width = <8>;
+		cap-mmc-highspeed;
+	};