kernel: mem_domain: extend sane_partition for non-overlapping regions

This commit extends the implementation of sane_partition(..) in
kernel/mem_domain.c so that it generates an ASSERT if partitions
inside a mem_domain overlap. This extension is only implemented
for the case when the MPU requires non-overlapping regions.

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