Unlock the field_register field with update hook and entity change.

Lots of the new features for the Commerce POS release on Drupal 8 list orders. A lot of the order listings require cashiers and register fields and these lists make the assumption that the field is 100% available on all order types. This is not the case and should be fixed in a follow up issue #2960807: Implicitly filter out orders from POS listings that don't have cashier or register fields..

Regardless of the "fix" this highlighted the "workaround" (which is to add these fields on other order types) is not possible because the register field is locked and cannot be added to other order bundles.