device: refactor to allow direct access to devicetree device structures

When using a devicetree node as an identifier we know the identifier
used to define the device structure.  This allows code to directly
reference that structure, avoiding the need to look it up by label at
runtime.

Change the macros so DEVICE_DT_* device objects are globally visible
using the node identifier as the object identifier.

Also add the necessary API to verify that a device that was captured
at build time successfully completed initialization and any other
steps necessary before it can be safely used.

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