kernel: mem_domain: fix warning in ASSERT expression

While fixing the ASSERT expressions in mem_domain.c to use
%lx instead of %x for uintptr_t variables, commit
f32330b2 has overlooked
one ASSERT expression specific to ARMv8-M. This causes
printk compilation warnings for ARMv8-M builds, so we
provide a fix here.

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