dt-bindings: arm: atmel: add missing samx7 to reset controller

Add this missing compatibility string to the Reset Controller
compatible string chip list.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>