[RFC PATCH] Support for big page sizes on 44x

[Ilya Yanok]

This adds support for selecting page size on PPC 44x.

However there are still number of problems:
1. We can't use default PKMAP_BASE definition with 64KB/256KB pages so we
change it. Not sure that it's optimal. Then redefined PKMAP_BASE is not
aligned on (1<<PMD_SHIFT), don't know if it is really bad.
2. with 16KB/64KB/256KB pages WARN_ON(!pmd_none(*pmd)) is triggered inside
dma_alloc_init() function. Not sure if it is really bad.
3. with 256KB pages ENTRIES_PER_PAGEPAGE in mm/shem.c become zero.
Second patch tries to address this issue.
4. Ugly ppc_page_asm.h file. I'd prefer to see these defines calculated
instead of being hardcoded but PTE_SHIFT and PMD_SHIFT are declared
inside #ifndef __ASSEMBLY__. Don't know why.

We would appreciate any comment.