arm: Fix L2 PTE access permission settings.

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's) 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.