Devicetree: Devicetree Bindings: Add new APIs

The existing APIs for enums were rewritten to support the changes.
Additional macros were added to now also support getting specific
indices from the array / string-array.
The macros can be evoked the same way like before, since nothing changed
for the interface. Therefore, these changes are backward compatible.

Signed-off-by: Joel Hirsbrunner <jhirsbrunner@baumer.com>