tests: code_relocation: support mr_canhubk3 boards

Add test scenario for NXP S32 platforms, for now only ported to
mr_canhubk3.

In some platforms the MPU region used for NULL pointer detection
conflicts with the ITCM region, causing access fault. Make disabling the
null pointer exception detection the default behavior for this scenario.

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