Optional properties:
- clock-names : Contains the names of the clocks:
"ssi_clk", for the core clock used to generate the external SPI clock.
- "pclk", the interface clock, required for register access.
+ "pclk", the interface clock, required for register access. If a clock domain
+ used to enable this clock then it should be named "pclk_clkdomain".
- cs-gpios : Specifies the gpio pins 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