scripts: edtlib: child binding compatibles match parents

Fixes: #29758

Commit 7165b77a ("scripts: edtlib:
refactor for first class bindings") introduced a Binding class.

Its child_binding instance attribute has a compatible which can be
None. Adjust this behavior so the child Binding object has the same
compatible as the parent binding which ultimately has a compatible.

Without this, sanitycheck's expr_parser is doing some matching on
compatibles in child nodes that is producing unexpected results.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>