dt-bindings: mtd: Document the STM32 QSPI bindings

* STMicroelectronics Quad Serial Peripheral Interface(QuadSPI)

Required properties:

- compatible: should be "st,stm32f469-qspi"

- reg: the first contains the register location and length.

       the second contains the memory mapping address and length

- reg-names: should contain the reg names "qspi" "qspi_mm"

- interrupts: should contain the interrupt for the device

- clocks: the phandle of the clock needed by the QSPI controller

- A pinctrl must be defined to set pins in mode of operation for QSPI transfer

Optional properties:

- resets: must contain the phandle to the reset controller.

A spi flash must be a child of the nor_flash node and could have some

properties. Also see jedec,spi-nor.txt.

Required properties:

- reg: chip-Select number (QSPI controller may connect 2 nor flashes)

- spi-max-frequency: max frequency of spi bus

Optional property:

- spi-rx-bus-width: see ../spi/spi-bus.txt for the description

Example:

qspi: spi@a0001000 {

	compatible = "st,stm32f469-qspi";

	reg = <0xa0001000 0x1000>, <0x90000000 0x10000000>;

	reg-names = "qspi", "qspi_mm";

	interrupts = <91>;

	resets = <&rcc STM32F4_AHB3_RESET(QSPI)>;

	clocks = <&rcc 0 STM32F4_AHB3_CLOCK(QSPI)>;

	pinctrl-names = "default";

	pinctrl-0 = <&pinctrl_qspi0>;

	flash@0 {

		reg = <0>;

		spi-rx-bus-width = <4>;

		spi-max-frequency = <108000000>;

		...

	};

};