arm64/mm: wire up CONFIG_ARCH_HAS_SET_DIRECT_MAP

Wire up the special helper functions to manipulate aliases of vmalloc
regions in the linear map.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>