dt-bindings: convert mtk-quadspi binding doc for spi-mtk-nor

spi-mtk-nor is a driver to replace mtk-quadspi and they have almost
the same device-tree bindings. Reuse this binding documentation and
convert it for new driver:

1. "Mediatek SoCs" -> "Mediatek ARM SoCs" because MTK MIPS SoCs
   use different controllers.
2. document "interrupts" as a required property because it's
   available on all SoCs with this controller and new driver takes
   advantages of it. It's implemented as optional only to maintain
   backward compatibility.
3. add a dummy interrupt binding in example.
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200306085052.28258-4-gch981213@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/mtd/mtk-quadspi.txt → Documentation/devicetree/bindings/spi/spi-mtk-nor.txt

-* Serial NOR flash controller for MediaTek SoCs
+* Serial NOR flash controller for MediaTek ARM SoCs
 
 Required properties:
 - compatible: 	  For mt8173, compatible should be "mediatek,mt8173-nor",
@@ -13,6 +13,7 @@ Required properties:
 		  "mediatek,mt7629-nor", "mediatek,mt8173-nor"
 		  "mediatek,mt8173-nor"
 - reg: 		  physical base address and length of the controller's register
+- interrupts:	  Interrupt number used by the controller.
 - 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,
@@ -22,20 +23,16 @@ Required properties:
 - #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
+There should be only one spi slave device following generic spi bindings.
+It's not recommended to use this controller for devices other than SPI NOR
+flash due to limited transfer capability of this controller.
 
 Example:
 
 nor_flash: spi@1100d000 {
 	compatible = "mediatek,mt8173-nor";
 	reg = <0 0x1100d000 0 0xe0>;
+	interrupts = <&spi_flash_irq>;
 	clocks = <&pericfg CLK_PERI_SPI>,
 		 <&topckgen CLK_TOP_SPINFI_IFR_SEL>;
 	clock-names = "spi", "sf";