Cadence Xtensa XTFPGA platform SPI controller.
  
  This simple SPI master controller is built into xtfpga bitstreams and is used
  to control daughterboard audio codec.
  
  Required properties:
  - compatible: should be "cdns,xtfpga-spi".
  - reg: physical base address of the controller and length of memory mapped
    region.