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>