scripts: edtlib: Forbid multiple bindings with the same compatible

Previously, if two bindings had the same 'compatible:'/'parent-bus:'
values, the binding that happened to be loaded last would get used.

Turn it into an error instead. This avoids tricking people into thinking
that bindings get loaded in a defined order.

Maybe overriding bindings could be allowed later, if we need it.

Fixes: #19536

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