kernel: mm: move kernel mm functions under kernel includes

This moves the k_* memory management functions from sys/ into
kernel/ includes, as there are kernel public APIs. The z_*
functions are further separated into the kernel internal
header directory.

Also made a quick change to doxygen to group sys_mem_* into
the OS Memory Management group so they will appear in doc.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>