soc: arm: nxp_kinetis: move NXP Kinetis SOCs to use SystemInit

Add call to SystemInit for all NXP Kinetis SOCs and remove any
redundant code from initialization flow. This allows watchdog
initialization to be removed from all Kinetis SOCs as it is handled
by SystemInit.

Since Kinetis watchdog is enabled by default at boot, allow watchdog
setup to by bypassed with CONFIG_WDOG_ENABLE_AT_BOOT. This
setting requires the user to provide a watchdog configuration hook
using z_arm_watchdog_init, but will allow the watchdog to remain
enabled.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>