ARM: at91/dt: define sam9n12ek crystal frequencies

Define sam9n12ek's main and slow crystal frequencies.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>