MAINTAINERS: Update Xilinx Maintainership

Peter C is leaving Xilinx, so update the maintainer list
to point to Alistair and Edgar from Xilinx and Peter's
personal email address.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 54b4c070452bac05aa3a9c1d75899bc097fef831.1436486024.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>