drivers: soc: atmel: add spdx license identifier

Add SPDX-License-Identifier.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
[nicolas.ferre@microhcip.com: remove license boilerplate now it's useless]
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/1611653376-24168-2-git-send-email-claudiu.beznea@microchip.com