device: introduce device flags

Introduce a new field to store device flags. Only device deferred init
flag has been added, replacing usage of linker hackery to know wether a
device requires initialization at boot time or not. This change will be
helpful in the near future as devices will become reference counted, so we
will need to know wether they have been initialized or not.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>