arm64: fix STRICT_MM_TYPECHECKS issue in PTE_CONT manipulation

The new page table code that manipulates the PTE_CONT flags does so
in a way that is inconsistent with STRICT_MM_TYPECHECKS. Fix it by
using the correct combination of __pgprot() and pgprot_val().

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>