aarch64: mmu: distinguish between device and normal memory

we need to do that, because load/store instructions
semantics change based on memory attributes.
In particular, we need Device-nGnRnE memory.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
work in progress

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>