Fix bugs and add missing functionalities in superpages support for ARMv6/v7

- Add large pages support in  pmap_is_referenced()
- Fix bug in write access fixup for section mappings
- Add helpful assertions
- Fix bug in pmap_pv_reclaim() regarding sections mappings
Put the complicated formula for the number of L2 PTEs into define

L2_PTE_NUM_TOTAL is a simple representation of
(L2_TABLE_SIZE_REAL / sizeof(pt_entry_t))