powerpc32: fix of PTE accessors and handle inverted HW bits via PTE accessors

Some powerpc like the 8xx do invert some PTE bits in HW. In order to avoid having to invert the bits each time we set them into hardware, this patch adds the invertion logic into the PTE accessors in order to get them already properly inversed in the tables.

Patchset:
1) powerpc32: misuse of accessors to pte_t objects
2) powerpc32: properly clear page table when 0 is not a good default PTE value
3) powerpc32: adds direct support to hardware inverted values in pte accessors
4) powerpc32/8xx: invert _PAGE_RW bit in PTE accessors

All changes have been successfully tested on MPC885

---
 arch/powerpc/include/asm/page.h          |  8 +++----
 arch/powerpc/include/asm/pgtable-ppc32.h | 37 +++++++++++++++++++++-----------
 arch/powerpc/include/asm/pgtable.h       | 17 ++++++++-------
 arch/powerpc/include/asm/pte-8xx.h       |  1 +
 arch/powerpc/include/asm/pte-common.h    |  3 +++
 arch/powerpc/kernel/head_8xx.S           |  3 ---
 arch/powerpc/mm/pgtable_32.c             | 14 ++++++++++++
 7 files changed, 56 insertions(+), 27 deletions(-)