This commit exposes k_mem_partition_attr_t outside User Mode, so
we can use struct k_mem_partition for defining memory partitions
outside the scope of user space (for example, to describe thread
stack guards or no-cacheable MPU regions). A requirement is that
the Zephyr build supports Memory protection. To signify this, a
new hidden, all-architecture Kconfig symbol is defined (MPU). In
the wake of exposing k_mem_partition_attr_t, the commit exposes
the MPU architecture-specific access permission attribute macros
outside the User space context (for all ARCHs), so they can be
used in a more generic way.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>