soc: arm: nxp kinetis: force custom fixed MPU region configuration

For the NXP Kinetis SoCs with the NXP MPU regions, we keep
the confiruation of the fixed SoC MPU regions at each SoC
definition.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>