ARM: at91/dt: Add Acme Arietta G25

Add a minimum Device Tree for Acme Arietta G25.
http://acme.systems/arietta



Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Sergio Tanzilli <tanzilli@acmesystems.it>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>