dt-bindings: dmaengine: Add MediaTek High-Speed DMA controller bindings

Document the devicetree bindings for MediaTek High-Speed DMA controller
which could be found on MT7623 SoC or other similar Mediatek SoCs.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/mtk-hsdma.txt

+MediaTek High-Speed DMA Controller
+==================================
+
+This device follows the generic DMA bindings defined in dma/dma.txt.
+
+Required properties:
+
+- compatible:	Must be one of
+		  "mediatek,mt7622-hsdma": for MT7622 SoC
+		  "mediatek,mt7623-hsdma": for MT7623 SoC
+- reg:		Should contain the register's base address and length.
+- interrupts:	Should contain a reference to the interrupt used by this
+		device.
+- clocks:	Should be the clock specifiers corresponding to the entry in
+		clock-names property.
+- clock-names:	Should contain "hsdma" entries.
+- power-domains: Phandle to the power domain that the device is part of
+- #dma-cells: 	The length of the DMA specifier, must be <1>. This one cell
+		in dmas property of a client device represents the channel
+		number.
+Example:
+
+        hsdma: dma-controller@1b007000 {
+		compatible = "mediatek,mt7623-hsdma";
+		reg = <0 0x1b007000 0 0x1000>;
+		interrupts = <GIC_SPI 98 IRQ_TYPE_LEVEL_LOW>;
+		clocks = <&ethsys CLK_ETHSYS_HSDMA>;
+		clock-names = "hsdma";
+		power-domains = <&scpsys MT2701_POWER_DOMAIN_ETH>;
+		#dma-cells = <1>;
+	};
+
+DMA clients must use the format described in dma/dma.txt file.