MAINTAINERS: Adopt the Gumstix computers-on-module machines

MAINTAINERS: Adopt the Gumstix computers-on-module machines

These COMs are hard to find, and the companie dropped the support
few years ago.

Per the "Gumstix Product Changes, Known Issues, and EOL" pdf:

- Phasing out: PXA270-based Verdex product line
  September 2012

- Phasing out: PXA255-based Basix & Connex
  September 2009

However there are still booting SD card image availables, very
convenient to stress test the QEMU SD card implementation.
Therefore I volunteer to keep an eye on this file, while it
is useful for testing.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180606144706.29732-1-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>