modules: lvgl: fix multiple LVGL inputs

The LVGL input callback does not use an instance specific name, so we
can only ever have one instance of this type. This issue was evident by
a build error, e.g., when using "zephyr,lvgl-keypad-input" nodes twice:

  error: redefinition of '_input_callback__lvgl_keypad_process_event'

Signed-off-by: Gero Schwäricke <gero.schwaericke@posteo.de>