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: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>