fix(pointing): Support multiple temp layer instances

[nobyk]

Previously, the input-processor-temp-layer driver failed to support multiple instances because the background work queue callback (layer_action_work_cb) was hardcoded to operate only on the first driver instance (instance 0). This caused actions from secondary instances to either be ignored or incorrectly apply state changes to the first instance.

This commit fixes the issue by:

• Adding a dev pointer to the layer_state_action struct to identify the originating device instance for each event.
• Updating the work callback to use the specific device instance from the action payload.
• Implementing a lookup mechanism for timeout-based deactivation to find and update the correct active instance managing the expiring layer.
• Fixing macro expansion errors in DT_INST_FOREACH_STATUS_OKAY usage.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[petejohanson]

Generally seems reasonable, just one minor comment on the mutex use. Thanks!