tests/bench_xtimer: update low memory boards list
tests/bench_xtimer: update low memory boards list

Samd21 based boards using the Arduino bootloader and stdio over usb by default doesn't have enough RAM to run the test in its default configuration (1000 timers)

yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,basilfx/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,basilfx/RIOT,ant9000/RIOT,yogo1212/RIOT,miri64/RIOT,OTAkeys/RIOT,kaspar030/RIOT,yogo1212/RIOT,miri64/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,yogo1212/RIOT,authmillenon/RIOT,ant9000/RIOT,basilfx/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,basilfx/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,kYc0o/RIOT,yogo1212/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT