Documentation: atmel-quadspi: add binding file for Atmel QSPI driver

This patch documents the DT bindings for the driver of the Atmel QSPI
controller embedded inside sama5d2x SoCs.
Signed-off-by: NCyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

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

+* Atmel Quad Serial Peripheral Interface (QSPI)
+
+Required properties:
+- compatible:     Should be "atmel,sama5d2-qspi".
+- reg:            Should contain the locations and lengths of the base registers
+                  and the mapped memory.
+- reg-names:      Should contain the resource reg names:
+                  - qspi_base: configuration register address space
+                  - qspi_mmap: memory mapped address space
+- interrupts:     Should contain the interrupt for the device.
+- clocks:         The phandle of the clock needed by the QSPI controller.
+- #address-cells: Should be <1>.
+- #size-cells:    Should be <0>.
+
+Example:
+
+spi@f0020000 {
+	compatible = "atmel,sama5d2-qspi";
+	reg = <0xf0020000 0x100>, <0xd0000000 0x8000000>;
+	reg-names = "qspi_base", "qspi_mmap";
+	interrupts = <52 IRQ_TYPE_LEVEL_HIGH 7>;
+	clocks = <&spi0_clk>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_spi0_default>;
+	status = "okay";
+
+	m25p80@0 {
+		...
+	};
+};