powerpc/mm/book3s-64: Move _PAGE_PTE to 2nd most significant bit

This changes _PAGE_PTE for 64-bit Book 3S processors from 0x1 to
0x4000_0000_0000_0000, because that bit is used as the L (leaf)
bit by PowerISA v3.0 CPUs in radix mode.  The "leaf" bit indicates
that the PTE points to a page directly rather than another radix
level, which is what the _PAGE_PTE bit means.

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