ARM: at91/dt: move sama5d3 SoC to the new main/slow clk model

Replace the old main and clk definitions (fixed rate clk) by the new main and
slow clk subtree definition (ck = mux(rc_osc, osc)).

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