arm64: fix incorrect use of pgprot_t variable

This fixes a build failure under STRICT_MM_TYPECHECKS, by adding
a missing pgprot_val() around a pgport_t reference.

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