ARM: Documentation: Update fec dts binding doc

    This patch update fec devicetree binding doc that add Optional
    properties "fsl,num-tx-queues" and "fsl,num-rx-queues".

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>