kernel: mm: rename k_mem_un/map_impl to k_mem_*_phys_guard

The internal functions k_mem_map_impl() and k_mem_unmap_impl()
are renamed to k_mem_map_phys_guard() and
k_mem_unmap_phys_guard() respectively to better clarify
their usage.

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