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

* 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 {

		...

	};

};