powerpc: Name xpn & x fields in HW Hash PTE format

Previously, the 36-bit code was using these bits, but they had
never been named in the pte format definition.  This patch just
gives those fields their proper names and adds a comment that
they are only present on some processors.

There is no functional code change.

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