ARM: dts: at91: add at91sam9260ek board DT

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: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>