arm: Fix L2 PTE access permissions management.

Keep following access permissions:

APX     AP     Kernel     User
 1      01       R         N
 1      10       R         R
 0      01      R/W        N
 0      11      R/W       R/W

Avoid using reserved in ARMv6 APX|AP settings:
- In case of unprivileged (user) access without permission to write,
  the access permission bits were being set to reserved for ARMv6
  (but valid for ARMv7) value of APX|AP = 111.

Fix-up faulting userland accesses properly:
- Wrong condition statement in pmap_fault_fixup() caused that
  any genuine, unprivileged access was being fixed-up instead of
  just skip doing anything and return. Staring from now we ensure
  proper reaction for illicit user accesses.
arm: Fix page reference emulation on ARMv6 and v7

In order to invalidate the page for reference emulation purposes
it is necessary to CLEAR the type bits in the page descriptor.