devicetree: basic support for enum tokens

Whenever a devicetree binding defines a string property whose
enumerated values are all tokenizable, generate C macros for each
property value that are the corresponding tokens.

Note that "token" is distinct from "identifier": both 'foo' and '123'
are valid tokens, but only 'foo' is a valid identifier. We permit some
strings which are not valid identifiers in anticipation that the
generalization may be useful, e.g. when defining macros that paste the
token onto a prefix that makes the whole thing an identifier.

Fixes: #21273
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>