[POWERPC] Expand RPN field to 34 bits when using 64k pages

The real page number field in our PTEs when configured for 64kB pages
is currently 32 bits, which turns out to be not quite enough for the
resources that the eHCA driver wants to map.  This expands the RPN
field to include 2 adjacent, previously-unused bits.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>