ARM: at91/dt: move at91sam9rl SoC to the new slow/main clock models

Move at91sam9rl SoC to the new main/slow clock model.

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