kernel: mm: rename z_phys_un/map to k_mem_*_phys_bare

This renames z_phys_map() and z_phys_unmap() to
k_mem_map_phys_bare() and k_mem_unmap_phys_bare()
respectively. This is part of the series to move memory
management functions away from the z_ namespace.

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