powerpc/mm: Unify PTE_RPN_SHIFT and _PAGE_CHG_MASK definitions

This updates the 32-bit headers to use the same definitions for the RPN
shift inside the PTE as 64-bit, and thus updates _PAGE_CHG_MASK to
become identical.

This does introduce a runtime visible difference, which is that now,
_PAGE_HASHPTE will be part of _PAGE_CHG_MASK and thus preserved. However
this should have no practical effect as it should have been preserved in
the first place and we got away with not having it there due to our
PTE access functions preserving it anyway.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>