KVM: arm64: Introduce addr_is_memory()

Introduce a helper usable in nVHE protected mode to check whether a
physical address is in a RAM region or not.

Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210809152448.1810400-16-qperret@google.com