ARM: at91/dt: sama5d3 crystals under the clocks node

Having clocks grouped in a subnode is common practice, so move the crystals
under a clocks node for the sama5d3 SoC and sama5d3 based boards.

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