AT91: Support for gsia18s board

The GS_IA18_S (GMS) is a carrier board from GeoSIG Ltd used with the
Stamp9G20 SoM from Taskit company.
It operate as an internet accelerometer.

Signed-off-by: Igor Plyatov <plyatov@gmail.com>
[nicolas.ferre@atmel.com: rm Kconfig, whitespace fixes, change machine name]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>