sparc64: add support for _PAGE_SPECIAL

Luckily there are still a few software PTE bits remaining and they even
match up in both the sun4u and sun4v pte layouts.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>