arm64: mm: Fix PMD_SECT_PROT_NONE definition

Modify the value of PMD_SECT_PROT_NONE to match that of PTE_NONE. This
should have been in commit 3676f9ef

 (Move PTE_PROT_NONE higher up).

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Cc: <stable@vger.kernel.org> # 3.11+: 3676f9ef

: arm64: Move PTE_PROT_NONE higher up
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>