SPI: Add vendor Atmel

Change-Id: I60e578003b857f5dcabb2e9bc75aa46acddb62b8
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7433
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)