powerpc8xx: Further optimisation of TLB handling

This patchset provides a further optimisation of TLB handling in the 8xx. Main changes are based on:
- Using processor handling of PGD/PTE Validity bits instead of testing ourselves the entries validity
- Aligning PGD address to allow direct bit manipulation
- Not saving registers like CR when not needed

It also adds support to any TASK_SIZE

Patchset:
01 - powerpc/8xx: remove remaining unnecessary code in FixupDAR 02 - powerpc/8xx: remove tests on PGDIR entry validity 03 - powerpc32: Use kmem_cache memory for PGDIR
04 - powerpc/8xx: Take benefit of aligned PGDIR
05 - powerpc/8xx: Optimise access to swapper_pg_dir
06 - powerpc/8xx: Remove duplicated code in set_context() 07 - powerpc/8xx: macro for handling CPU15 errata
08 - powerpc/8xx: Handle CR out of exception PROLOG/EPILOG 09 - powerpc/8xx: dont save CR in SCRATCH registers
10 - powerpc/8xx: Use SPRG2 instead of DAR for saving r3 11 - powerpc/8xx: Add support for TASK_SIZE greater than 0x80000000

All changes have been successfully tested on MPC885

---
 arch/powerpc/include/asm/pgtable-ppc32.h |   4 +
 arch/powerpc/kernel/head_8xx.S           | 197 +++++++++++++++----------------
 arch/powerpc/mm/pgtable_32.c             |  16 ++-
 3 files changed, 111 insertions(+), 106 deletions(-)