Bluetooth: Mesh: RFU fixed group addresses can be used for subscription

Mesh 1.0.1 specification doesn't prohibit to use full range of
the fixed group addresses for subscription.
The immediate use case is upcoming IPT transport models
which will also work on Mesh 1.0 devices. The models
declare their own fixed group addresses.

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