Claude review: dt-bindings: display: panel: Add Himax HX83121A

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Missing dual-DSI port schema.** This is a dual-DSI panel (as the example and driver show), yet the binding doesn't reference `panel-common-dual.yaml` or define a `ports` schema at all. Compare with `novatek,nt36523.yaml` which uses:
```yaml
allOf:
  - $ref: panel-common-dual.yaml#
```
Without this, `dt_binding_check` likely won't validate the `ports` node in the example, and the `port@1` in the example is undocumented. The `additionalProperties: false` should probably become `unevaluatedProperties: false` when using the `$ref`.

**Missing `port` property.** The example has a `ports` node with two ports, but there is no `port` or `ports` property declared in the schema. This is handled by the `panel-common-dual.yaml` ref, but since it's not referenced, this is an omission.

**`avdd-supply` and `avee-supply` not required.** They are listed as optional in the binding but the driver always requests them via `regulator_bulk_get`. If the hardware always needs them, consider making them required, or at minimum document that they are optional (which they currently are, as regulators can be dummy). This is minor - the current approach is acceptable if dummy regulators suffice.

**Minor style in example:** `port@1{` is missing a space before `{` (line 326 of the binding yaml). Should be `port@1 {`. This exists in the nt36523 binding too, but it's still poor style.

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