Documentation: mtk-quadspi: update DT bindings

* Serial NOR flash controller for MTK MT81xx (and similar)

Required properties:

- compatible: 	  should be "mediatek,mt8173-nor";

- compatible: 	  The possible values are:

		  "mediatek,mt2701-nor"

		  "mediatek,mt7623-nor"

		  "mediatek,mt8173-nor"

		  For mt8173, compatible should be "mediatek,mt8173-nor".

		  For every other SoC, should contain both the SoC-specific compatible string

		  and "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