arm: xlnx-zynqmp: Add UART support

arm: xlnx-zynqmp: Add UART support

There are 2x Cadence UARTs in Zynq MP. Add them.

Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: e30795536f77599fabc1052278d846ccd52322e2.1431381507.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>