STM32 QSPI controller device tree bindings
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Required properties:
- compatible		: should be "st,stm32-qspi".
- reg			: 1. Physical base address and size of SPI registers map.
			  2. Physical base address & size of mapped NOR Flash.
- spi-max-frequency	: Max supported spi frequency.
- status		: enable in requried dts.

Connected flash properties
--------------------------
- spi-max-frequency	: Max supported spi frequency.
- spi-tx-bus-width	: Bus width (number of lines) for writing (1-4)
- spi-rx-bus-width	: Bus width (number of lines) for reading (1-4)
- memory-map		: Address and size for memory-mapping the flash

Example:
	qspi: quadspi@A0001000 {
		compatible = "st,stm32-qspi";
		#address-cells = <1>;
		#size-cells = <0>;
		reg = <0xA0001000 0x1000>, <0x90000000 0x10000000>;
		reg-names = "QuadSPI", "QuadSPI-memory";
		interrupts = <92>;
		spi-max-frequency = <108000000>;
		status = "okay";

		qflash0: n25q128a {
			#address-cells = <1>;
			#size-cells = <1>;
			compatible = "micron,n25q128a13", "spi-flash";
			spi-max-frequency = <108000000>;
			spi-tx-bus-width = <4>;
			spi-rx-bus-width = <4>;
			memory-map = <0x90000000 0x1000000>;
			reg = <0>;
		};
	};