ARM: at91/dt: add basic dtsi for sama5d2 SoC

Only the basic support for this new Atmel Cortex-A5 SoC. A subset of the
peripherals is setup to allow booting.
IRQ, clocks, USB, crypto, timers, rtc, ethernet, spi/i2c and
uart/usart peripheral nodes are added.
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>