dt-bindings: atmel-sysreg: add microchip,sam9x60-ddramc

Add microchip,sam9x60-ddramc to DT bindings documentation.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-10-git-send-email-claudiu.beznea@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>