dt-bindings: dma: uart: add uart dma bindings

add uart dma bindings
Signed-off-by: Long Cheng <long.cheng@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/dma/8250_mtk_dma.txt

+* Mediatek UART APDMA Controller
+
+Required properties:
+- compatible should contain:
+  * "mediatek,mt2712-uart-dma" for MT2712 compatible APDMA
+  * "mediatek,mt6577-uart-dma" for MT6577 and all of the above
+
+- reg: The base address of the APDMA register bank.
+
+- interrupts: A single interrupt specifier.
+
+- clocks : Must contain an entry for each entry in clock-names.
+  See ../clocks/clock-bindings.txt for details.
+- clock-names: The APDMA clock for register accesses
+
+Examples:
+
+	apdma: dma-controller@11000380 {
+		compatible = "mediatek,mt2712-uart-dma";
+		reg = <0 0x11000380 0 0x400>;
+		interrupts = <GIC_SPI 63 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 64 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 65 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 66 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 67 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 68 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 69 IRQ_TYPE_LEVEL_LOW>,
+			     <GIC_SPI 70 IRQ_TYPE_LEVEL_LOW>;
+		clocks = <&pericfg CLK_PERI_AP_DMA>;
+		clock-names = "apdma";
+		#dma-cells = <1>;
+	};
+