Documentation: DT: cpsw: remove rx_descs property

There is no reason to hold s/w dependent parameter in device tree.
Even more, there is no reason in this parameter because davinici_cpdma
driver splits pool of descriptors equally between tx and rx channels
anyway.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>