dt-bindings: mtd: Document the STM32 QSPI bindings

This patch adds documentation of device tree bindings for the STM32
QSPI controller.
Signed-off-by: NLudovic Barre <ludovic.barre@st.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/stm32-quadspi.txt

+* 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>;
+		...
+	};
+};