ARM: at91: Add DT description files for AT91SAM9N12-EK

Added AT91SAM9N12 SoC DT file, as well as the board definition
.dts file for AT91SAM9N12-EK.

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