cmake: Add REQUIRED option to devicetree getters

namely:
 - dt_nodelabel
 - dt_alias
 - dt_prop

these currently fail quietly.
This can cause some very confusing errors later on.
By adding `REQUIRED` to the function call one can
easily generate a more clear message:

    required nodelabel not found: ...

Signed-off-by: David van Rijn <david@refractor.dev>