The MIPS implementation of transparent huge-pages (THP) is 64-bit only,
and of course also requires that the CPU supports huge-pages.

Currently it's entirely possible to enable THP in other configurations,
which then fail to build due to pfn_pmd() not being defined.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: linux-mips@linux-mips.org
Acked-by: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/4972/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>