ARM: at91/dt: declare sckc node on at91sam9g45

Declare the SCKC (Slow Clock Configuration) block and its clks.
Make use of the clk32k clk instead of slow_osc where appropriate.

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>