samples: lvgl: Extend hello world sample button functionality

This commit adds a callback to the button to reset the counter when the
button is pressed. The button is automatically created when lvgl
pointer or kscan support is enabled. The same reset functionality was
already available for possible gpio button.

Please make sure to define a zephyr input device in device
tree (e.g. "zephyr,lvgl-pointer-input").

Signed-off-by: Martin Kiepfer <m.kiepfer@teleschirm.org>