spi: document tx/rx clock delay properties

Recommended properties:

- spi-max-frequency: Definition as per

                     Documentation/devicetree/bindings/spi/spi-bus.txt

Optional properties:

- nvidia,tx-clk-tap-delay: Delays the clock going out to the external device

  with this tap value. This property is used to tune the outgoing data from

  Tegra SPI master with respect to outgoing Tegra SPI master clock.

  Tap values vary based on the platform design trace lengths from Tegra SPI

  to corresponding slave devices. Valid tap values are from 0 thru 63.

- nvidia,rx-clk-tap-delay: Delays the clock coming in from the external device

  with this tap value. This property is used to adjust the Tegra SPI master

  clock with respect to the data from the SPI slave device.

  Tap values vary based on the platform design trace lengths from Tegra SPI

  to corresponding slave devices. Valid tap values are from 0 thru 63.

Example:

spi@7000d600 {

	reset-names = "spi";

	dmas = <&apbdma 16>, <&apbdma 16>;

	dma-names = "rx", "tx";

	<spi-client>@<bus_num> {

		...

		...

		nvidia,rx-clk-tap-delay = <0>;

		nvidia,tx-clk-tap-delay = <16>;

		...

	};

};