Documentation: add the binding file for Freescale QuadSPI driver

* Freescale Quad Serial Peripheral Interface(QuadSPI)

Required properties:

  - compatible : Should be "fsl,vf610-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 "QuadSPI" and "QuadSPI-memory"

  - interrupts : Should contain the interrupt for the device

  - clocks : The clocks needed by the QuadSPI controller

  - clock-names : the name of the clocks

Optional properties:

  - fsl,qspi-has-second-chip: The controller has two buses, bus A and bus B.

                              Each bus can be connected with two NOR flashes.

			      Most of the time, each bus only has one NOR flash

			      connected, this is the default case.

			      But if there are two NOR flashes connected to the

			      bus, you should enable this property.

			      (Please check the board's schematic.)

Example:

qspi0: quadspi@40044000 {

	compatible = "fsl,vf610-qspi";

	reg = <0x40044000 0x1000>, <0x20000000 0x10000000>;

	reg-names = "QuadSPI", "QuadSPI-memory";

	interrupts = <0 24 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&clks VF610_CLK_QSPI0_EN>,

		<&clks VF610_CLK_QSPI0>;

	clock-names = "qspi_en", "qspi";

	flash0: s25fl128s@0 {

		....

	};

};