ARM: at91: add defconfig for device tree

This will enable all current SoC support on DT (9260, 9g20, 9g45 family
and 9x5 family).

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