MAINTAINERS: Add new qemu-arm mailing list to ARM related entries

We now have a qemu-arm mailing list for ARM patches and discussion,
so add an L: entry for it to the various ARM related entries in
MAINTAINERS.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1446129661-5239-1-git-send-email-peter.maydell@linaro.org