dts: nxp_k2x: Correct and enable MCG clock controller binding

nxp_k2x.dtsi: Corrects Multipurpose Clock Generator binding
to utilize nxp,kinetis-mcg. Assign MCG to FlexTimer devices.
Enable FlexTimer clocks via System Integration Module clock
gate control registers.

Kconfig.defconfig.mk22f12: Enable default use of
CLOCK_CONTROL_MCUX_MCG when CLOCK_CONTROL is selected.

Signed-off-by: Ryan Holleran <rhollerar@gmail.com>