arm64: Use the generic devmem_is_allowed()

I recently copied this into lib/ for use by the RISC-V port.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>