DesignWare SPI device

Required properties:
- compatible : should be "snps,dw-spi-mmio" or "snps,dw-spi-pci"
- reg : offset and length of the register set for the device
- interrupts: The interrupt number to the cpu. The interrupt specifier format
  depends on the interrupt controller.
- num-chipselect: Contains the number of the chipselect
- bus-num: Bus number

Example:
		spi: spi@fff00000 {
				compatible = "snps,dw-spi-mmio";
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0xfff00000 0x1000>;
				interrupts = <0 154 4>;
				num-chipselect = <4>;
				bus-num = <0>;
		};
