MAINTAINERS: Add ZynqMP to MAINTAINERS file

Add the Xilinx ZynqMP SoC and EP108 machine to the maintainers
file.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: fed078103a0b02cfb3adadbe8e80e4420d554505.1436486024.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>