device: add API to check whether a device is ready to use

Currently this is useful only for some internal applications that
iterate over the device table, since applications can't get access to
a device that isn't ready, and devices can't be made unready.  So it's
introduced as internal API that may be exposed as device_ready() when
those conditions change.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>