AT91: add board support for Pcontrol_G20

Board is a carrier board for Stamp9G20, with additional peripherals
for a building automation system

Signed-off-by: Peter Gsellmann <pgsellmann@portner-elektronik.at>
[nicolas.ferre@atmel.com: remove machine_desc.io_pg_offst and .phys_io]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>