examples/lvgldemo: Add optional touchscreen diagnostics.

[aviralgarg05]

Note: Please adhere to Contributing Guidelines.

Summary

Add an opt-in LVGL touchscreen diagnostic screen and make the framebuffer
device path configurable for framebuffer-backed targets.

The default demo selection and LCD path are unchanged. When
CONFIG_EXAMPLES_LVGLDEMO_TOUCH_DIAGNOSTIC is enabled, the screen reports
whether the input device opened and displays live press state and
coordinates. This separates driver-availability problems from coordinate
mapping problems without requiring serial-only diagnostics.

Impact

• New feature: YES, opt-in diagnostic mode.
• User adaptation: NO; existing configurations retain current behavior.
• Build: YES only when the new option is selected.
• Hardware: NO driver change.
• Documentation: YES; Kconfig help documents the options.
• Security: NO new security boundary or privilege change.
• Compatibility: Existing LCD and demo paths remain unchanged.
• Other: NO.

Testing

• Build host: macOS 26.5, arm64, GCC 14.2.0.
• Target: Xtensa ESP32-S3, Waveshare ESP32-S3-Touch-LCD-7.
• git diff --check
• NuttX tools/checkpatch.sh
• ESP32-S3 framebuffer/touchscreen build
• Physical touchscreen press/coordinate observation on /dev/input0
• Partial LVGL initialization is released on display-open failure

Testing logs before change:

The base configuration has no opt-in touchscreen diagnostic screen.

Testing logs after change:

/dev/fb0 and /dev/input0 opened on the ESP32-S3 target.
Live touch press state and coordinates were observed on the display.

PR verification Self-Check

• This PR contains one focused diagnostic change.
• I have updated all required description fields above.
• I have reviewed and signed the commit.
• The branch passes local style checks.
• This PR adheres to the current contribution and coding guidelines.
• My PR is still work in progress.
• My PR is ready for review and can be safely merged.

Rebased onto current master (2aebae740); the diff is limited to
examples/lvgldemo/.