cadence_gem: Add the num-priority-queues property

cadence_gem: Add the num-priority-queues property

The Cadence GEM hardware supports N number priority queues, this patch is a
step towards that by adding the property to set the queues. At the moment
behaviour doesn't change as we only use queue 0.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 6543ec0d0c4bfd2678d0ed683efb197e91b17733.1469727764.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>