dt-bindings: snps,dw-apb-ssi: Add optional clock bindings documentation

- 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.

- clocks : phandle for the core clock used to generate the external SPI clock.

- clocks : phandles for the clocks, see the description of clock-names below.

   The phandle for the "ssi_clk" is required. The phandle for the "pclk" clock

   is optional. If a single clock is specified but no clock-name, it is the

   "ssi_clk" clock. If both clocks are listed, the "ssi_clk" must be first.

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.

- 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