Bluetooth: Mesh: subscription on fixed group addresses

There is errata clarification (Errata ID:18700)
about subscriptions on fixed group addresses.
It is possible to subscribe models on non primary elements
on any fixed group address except all nodes address.
Devices should be able to receive messages on fixed addresses
even if they do not support the feature
to which the fixed group address belongs.

Signed-off-by: Aleksandr Khromykh <Aleksandr.Khromykh@nordicsemi.no>