edtlib: implement copy.deepcopy() for EDT

Just like we did for dtlib in 15e3e317
("dtlib: implement copy.deepcopy() for DT"), except this time it's for
EDT. This also can do no harm and will be useful for implementing
system devicetree support.

No functional changes expected under the assumption that no users are
relying on us having stashed the exact bindings_dirs list passed to
the constructor. This patch switches to making a defensive copy, which
is safer and makes implementing this a little cleaner.

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