ARM: dts: at91: sama5d3: switch to new sckc bindings

Remove the child nodes of the sckc as they are not necessary anymore.

Also, switch to the new atmel,sama5d3-sckc compatible string to use the
proper startup time for the RC oscillator (500 µs instead of 75).

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>