soc: nxp: kv5x: move clk divider options to device tree

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

The kv5x device tree originally used the undefined
"nxp,kv58-mcg" binding for the MCG node. This has been
replaced by the general "nxp,kinetis-mcg" binding.

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