docs: dt: writing-schema: explain additional/unevaluatedProperties

  A list of DT properties from the 'properties' section that

  must always be present.

additionalProperties / unevaluatedProperties

  Keywords controlling how schema will validate properties not matched by this

  schema's 'properties' or 'patternProperties'. Each schema is supposed to

  have exactly one of these keywords in top-level part, so either

  additionalProperties or unevaluatedProperties. Nested nodes, so properties

  being objects, are supposed to have one as well.

  * additionalProperties: false

      Most common case, where no additional schema is referenced or if this

      binding allows subset of properties from other referenced schemas.

  * unevaluatedProperties: false

      Used when this binding references other schema whose all properties

      should be allowed.

  * additionalProperties: true

      Rare case, used for schemas implementing common set of properties. Such

      schemas are supposed to be referenced by other schemas, which then use

      'unevaluatedProperties: false'.  Typically bus or common-part schemas.

examples

  Optional. A list of one or more DTS hunks implementing the

  binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.