dts: edtlib: Sanity-check contents of 'sub-node:'

The contents of 'sub-node:' was assigned as-is as the binding, bypassing
_check_binding(). This also hid an error in
test-bindings/sub-node-parent.yaml.

Require 'sub-node:' to just have 'properties:' in it, and sanity-check
the properties like for regular bindings.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>