edtlib: support inferring binding from node content

Clean up of devicetree tooling removed generation of information
present in devicetree in nodes that have no compatible, or for extra
properties not defined by a binding.  Discussion proposed that these
properties should be allowed, but only in a defined node /zephyr,user.
For that node infer bindings based on the presence of properties.

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