arm: nxp_mpu: enable module's clock only when needed

NXP SYSMPU is used in other SoCs besides the Kinetis series. For
devices like S32K1xx, its bus interface clock lacks of clock gating
and it's driven by the system clock. Hence, only enable the module
clock for the Kinetis series.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>