device: add a DEVICE_DT_GET_BY_IDX macro

Add a DEVICE_DT_GET_BY_IDX macro to get a struct device reference for a
phandles node. This can be used directly as an argument of macros such
as DT_INST_FOREACH_PROP_ELEM_SEP.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>