ARM: at91: add at91sam9260 DT support

The at91sam9260 and at91sam9g20 share most of the same IP.
So udpate the node property in the at91sam9g20 only.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>