soc: nxp: kinetis: kl2x: move watchdog init to z_arm_watchdog_init

Move the NXP Kinetis KL2x watchdog initialization code to
z_arm_watchdog_init() and make it optional based on
CONFIG_WDOG_INIT.

This brings the KL2x in line with the other NXP Kinetis series SoC
implementations.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>