soc: arm: nxp: kinetis: disable watchdog init if bootloader is enabled

Disable the early watchdog initialization for the NXP Kinetis series
if the application is to be chain-loaded by mcuboot.

The early watchdog initialization must only take place once and needs
to happen within a SoC specific number of CPU clock cycles after
reset.

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