scripts: edtlib: Avoid modifying the global yaml.(C)Loader

edtlib is a library, and modifying yaml.(C)Loader directly interferes
with any binding loading in edtlib clients. To avoid that, add a custom
loader for bindings.

Internally, PyYAML does this, which is why defining a separate class
works:

    @classmethod
    def add_constructor(cls, tag, constructor):
        if not 'yaml_constructors' in cls.__dict__:
            cls.yaml_constructors = cls.yaml_constructors.copy()
        cls.yaml_constructors[tag] = constructor

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