Realtek DesignWare SSI Interface.

Required properties:
- compatible : "realtek,dw-apb-ssi"
- reg : The register base for the controller.
- interrupts : Should contain the SSI interrupt number.
- #address-cells : <1>, as required by generic SPI binding.
- #size-cells : <0>, also as required by generic SPI binding.
- clocks : Clock for ssi controller

Optional properties:
- 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).
- interrupt-parent: Should be the phandle for the interrupt controller

Child nodes as per the generic SPI binding.

Example:

	spi@0x18830000 {
		compatible = "realtek,dw-apb-ssi";
		reg = <0x18830000 0x10000>;
		interrupts = <BSP_IRQ_OTHERS>;
		#address-cells = <1>;
		#size-cells = <0>;
		num-cs = <2>;
	};
