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

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

Both the kw40z and kw41z device tree originally used an
undefined "nxp,kw41z-mcg" binding for the MCG node.
This has been replaced with the general "nxp,kinetis-mcg"
binding instead.

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