ARM: at91: dt: sam9rl: Device Tree for the at91sam9rlek

Add a device tree for the at91sam9rl-ek. For now it supports:
 - MMC
 - dbgu
 - usart1
 - watchdog
 - nand
 - leds
 - buttons

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>