doc: dt: mtd: add documentation for Mediatek spi-nor controller

Add device tree binding documentation for serial flash with
Mediatek serial flash controller
Signed-off-by: Bayi Cheng <bayi.cheng@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
[Brian: fixed up language]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/mtk-quadspi.txt

+* Serial NOR flash controller for MTK MT81xx (and similar)
+
+Required properties:
+- compatible: 	  should be "mediatek,mt8173-nor";
+- reg: 		  physical base address and length of the controller's register
+- clocks: 	  the phandle of the clocks needed by the nor controller
+- clock-names: 	  the names of the clocks
+		  the clocks should be named "spi" and "sf". "spi" is used for spi bus,
+		  and "sf" is used for controller, these are the clocks witch
+		  hardware needs to enabling nor flash and nor flash controller.
+		  See Documentation/devicetree/bindings/clock/clock-bindings.txt for details.
+- #address-cells: should be <1>
+- #size-cells:	  should be <0>
+
+The SPI flash must be a child of the nor_flash node and must have a
+compatible property. Also see jedec,spi-nor.txt.
+
+Required properties:
+- compatible:	  May include a device-specific string consisting of the manufacturer
+		  and name of the chip. Must also include "jedec,spi-nor" for any
+		  SPI NOR flash that can be identified by the JEDEC READ ID opcode (0x9F).
+- reg :		  Chip-Select number
+
+Example:
+
+nor_flash: spi@1100d000 {
+	compatible = "mediatek,mt8173-nor";
+	reg = <0 0x1100d000 0 0xe0>;
+	clocks = <&pericfg CLK_PERI_SPI>,
+		 <&topckgen CLK_TOP_SPINFI_IFR_SEL>;
+	clock-names = "spi", "sf";
+	#address-cells = <1>;
+	#size-cells = <0>;
+	status = "disabled";
+
+	flash@0 {
+		compatible = "jedec,spi-nor";
+		reg = <0>;
+	};
+};
+