edtlib: improve enum support

Add enum_tokenizable and enum_upper_tokenizable to PropertySpec. These
allow a PropertySpec to declare that it both has an enumeration of
values and all of them are strings which are "tokenizable". Don't
bother extending Property with these; the user can access the
information through Property.spec now, so the extra delegation is
unnecessary.

See the docstrings for details on what "tokenizable" means. The basic
idea is that we should be able to use the DT binding's enum values as
C 'enum' enumerators in a "reasonable way".

Add val_as_token to Property. This produces a canonical token for the
property value.

Add tests for this feature in particular and property enumerations in
general.

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