powerpc/mm/book3s-64: Shuffle read, write, execute and user bits in PTE

This moves the _PAGE_EXEC, _PAGE_RW and _PAGE_USER bits around in
the Linux PTE on 64-bit Book 3S systems to correspond with the bit
positions used in radix mode by PowerISA v3.0 CPUs.  This also adds
a _PAGE_READ bit corresponding to the read permission bit in the
radix PTE.  _PAGE_READ is currently unused but could possibly be used
in future to improve pte_protnone().

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>