lvgldemo: allow to customize input dev path

[XuNeo]

Summary

Default to /dev/input0, now it's configurable.

Impact

No functionality change after this change. Impact is minor.

Testing

Tested with sim:lvgl_fb.

[nuttxpr]

[Experimental Bot, please feedback here]

This Pull Request is missing crucial information required by the NuttX PR requirements.

Here's what's missing and needs to be addressed:

Summary:

• Why is this change necessary? Is it a fix for a bug? Are you adding a new feature to allow users to configure the input device? Clearly state the reason for the change.
• What functional part of the code is being changed? Be specific. Mention the file(s), modules, or subsystems affected by the change.
• How does the change exactly work? Provide a technical explanation. If you've added configuration options, explain how to use them.

Impact:

• You need to be more specific for all "NO" answers. Just saying "NO" is not sufficient. For example, instead of "Impact on build (will build process change)? NO", say "Impact on build: The build process remains unchanged." If an item is a "YES", then you MUST provide a description.
• Target(s): Since you've tested with sim:lvgl_fb, specify what architecture the simulator is running on (e.g., sim:qemu-arm).

Testing:

• Provide actual testing logs (before and after the change). This is essential to demonstrate the issue the PR addresses and to prove the fix works.

To improve your PR:

1. Expand on the missing information detailed above.
2. Clearly link your PR to any related NuttX issues. If there's no related issue, consider creating one to explain the motivation behind the change.

By providing complete and detailed information, your PR will be easier to review, understand, and ultimately merge into NuttX.