soc: nxp: kl2x: move clk divider options to device tree

Use kinetis SIM clock divider options set in device tree
instead of hardcoded values.

The kl25z device tree did not previously define a MCG node.
This has now been added with the general "nxp,kinetis-mcg"
binding.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@prevas.dk>