Add Device Tree source file for at91sam9260ek board. This official Atmel
Evaluation Kit is designed around a SoC based on a ARM 926 core the
at91sam9260.

The board is also added to the dts Makefile.
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>