spi: document tx/rx clock delay properties

Tegra SPI controller has TX and RX trimmers to tuning the delay of
SPI master clock with respect to the data.

TX and RX tap values are based on the platform validation across the
PVT and the trimmer values vary based on the trace lengths to the
corresponding SPI devices.

Signed-off-by: Sowjanya Komatineni <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/nvidia,tegra114-spi.txt

···
 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>;
+		...
+	};
+
 };