Documentation/devicetree: document cavium-pip rx-delay/tx-delay properties

Document cavium-pip rx-delay/tx-delay properties. Currently the board
specific values need to be hardcoded in the platform code, which we
want to avoid when moving to DT-only booting.
Signed-off-by: NAaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/cavium-pip.txt

@@ -37,6 +37,12 @@ Properties for PIP port which is a child the PIP interface:
 
 - phy-handle: Optional, see ethernet.txt file in the same directory.
 
+- rx-delay: Delay value for RGMII receive clock. Optional. Disabled if 0.
+  Value range is 1-31, and mapping to the actual delay varies depending on HW.
+
+- tx-delay: Delay value for RGMII transmit clock. Optional. Disabled if 0.
+  Value range is 1-31, and mapping to the actual delay varies depending on HW.
+
 Example:
 
 	pip@11800a0000000 {