xtensa: mmu: rename prefix z_xtensa to xtensa_mmu

This follows the idea to remove any z_ prefix. Since MMU has
a large number of these, separate out these changes into one
commit to ease review effort.

Since these are no longer have z_, these need proper doxygen
doc. So add them too.

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