kernel: mm: introduce k_mem_phys_map()/_unmap()

This is similar to k_mem_map()/_unmap(). But instead of using
anonymous memory, the provided physical region is mapped
into virtual address instead. In addition to simple mapping
physical ro virtual addresses, the mapping also adds two
guard pages before and after the virtual region to catch
buffer under-/over-flow.

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