xtensa: fix TLB multihit exceptions

- set _PAGE_USER in the pte_clear to avoid having TLB multihit exceptions
  (see following threads for more details);
  http://lists.linux-xtensa.org/pipermail/linux-xtensa/Week-of-Mon-20130401/
  http://lists.linux-xtensa.org/pipermail/linux-xtensa/Week-of-Mon-20130408/
- improved documentation of the PTE layout
- fix PTE mapping for present and 'prot_none' pages for T1050 hw and earlier
- fix pte_file offset and size
- add check for the correct number of bits for swap type

CC: piet.delaney@gmail.com
CC: jcmvbkbc@gmail.com
Signed-off-by: Chris Zankel <chris@zankel.net>