kernel: mem_domain: optimize sane partition checking

This commit optimizes the process of checking that the
added partitions in a mem_domain are sane. It places the
sane_partition checking inside the loop of adding the
partitions in the mem_domain, so that the checkings are
not performed twice, and no partition is checked against
itself.

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