MIPS: Fix build failure by adding definition of pfn_pmd().

MIPS: Fix build failure by adding definition of pfn_pmd().

With CONFIG_TRANSPARENT_HUGEPAGE=y and CONFIG_HUGETLBFS=y we get the
following build failure:

  CC      mm/huge_memory.o
mm/huge_memory.c: In function 'set_huge_zero_page':
mm/huge_memory.c:780:2: error: implicit declaration of function 'pfn_pmd' [-Werror=implicit-function-declaration]
mm/huge_memory.c:780:8: error: incompatible types when assigning to type 'pmd_t' from type 'int'

Add a definition of pfn_pmd() for 64-bit kernels (the only place huge
pages are currently supported).

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4813/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>