Synopsys DesignWare AMBA 2.0 Synchronous Serial Interface.

Required properties:
- compatible : "snps,dw-apb-ssi"
- reg : The register base for the controller.
- interrupts : One interrupt, used by the controller.
- #address-cells : <1>, as required by generic SPI binding.
- #size-cells : <0>, also as required by generic SPI binding.

Optional properties:
- cs-gpios : Specifies the gpio pis to be used for chipselects.
- num-cs : The number of chipselects. If omitted, this will default to 4.
- reg-io-width : The I/O register width (in bytes) implemented by this
  device.  Supported values are 2 or 4 (the default).

Child nodes as per the generic SPI binding.

Example:

	spi@fff00000 {
		compatible = "snps,dw-apb-ssi";
		reg = <0xfff00000 0x1000>;
		interrupts = <0 154 4>;
		#address-cells = <1>;
		#size-cells = <0>;
		num-cs = <2>;
		cs-gpios = <&gpio0 13 0>,
			   <&gpio0 14 0>;
	};